Improve and extend documentation of ElDoc

* lisp/emacs-lisp/eldoc.el (eldoc-documentation-strategy)
(eldoc-documentation-enthusiast)
(eldoc-documentation-compose-eagerly)
(eldoc-documentation-compose, eldoc-documentation-default)
(eldoc-minibuffer-message, eldoc-idle-delay)
(eldoc-print-after-edit)
(eldoc-echo-area-display-truncation-message)
(eldoc-echo-area-use-multiline-p)
(eldoc-echo-area-prefer-doc-buffer): Doc string fixes.

* doc/emacs/emacs.texi (Top):
* doc/emacs/programs.texi (Documentation, Programming Language Doc):
Rename "Lisp Doc" to "Programming Language Doc", including in
parent menus.  All references changed.
(Programming Language Doc): Formerly "Lisp Doc".  Rewrite to not
be specific to Emacs Lisp.  Improve markup and wording.  Document
more commands and variables.
* doc/emacs/maintaining.texi (Maintaining): Improve indexing.

9 files changed, 194 insertions, 103 deletions

diff --git a/doc/emacs/custom.texi b/doc/emacs/custom.texi
index 6e5a0ddc1c7..08ada2a70bb 100644
--- a/doc/emacs/custom.texi
+++ b/doc/emacs/custom.texi
@@ -1271,7 +1271,7 @@ mode: my-new
 disable a minor mode in a local variables list, use the @code{eval}
 keyword with a Lisp expression that runs the mode command
 (@pxref{Minor Modes}).  For example, the following local variables
-list enables ElDoc mode (@pxref{Lisp Doc}) by calling
+list enables ElDoc mode (@pxref{Programming Language Doc}) by calling
 @code{eldoc-mode} with no argument (calling it with an argument of 1
 would do the same), and disables Font Lock mode (@pxref{Font Lock}) by
 calling @code{font-lock-mode} with an argument of @minus{}1.
diff --git a/doc/emacs/emacs.texi b/doc/emacs/emacs.texi
index 727f5f93bff..5c81641bf63 100644
--- a/doc/emacs/emacs.texi
+++ b/doc/emacs/emacs.texi
@@ -718,7 +718,7 @@ Documentation Lookup
 
 * Info Lookup::        Looking up library functions and commands in Info files.
 * Man Page::           Looking up man pages of library functions and commands.
-* Lisp Doc::           Looking up Emacs Lisp functions, etc.
+* Programming Language Doc:: Looking up program functions, variables, etc.
 
 C and Related Modes
 
diff --git a/doc/emacs/maintaining.texi b/doc/emacs/maintaining.texi
index 94171b3a089..2de6b3604f8 100644
--- a/doc/emacs/maintaining.texi
+++ b/doc/emacs/maintaining.texi
@@ -4,6 +4,8 @@
 @c See file emacs.texi for copying conditions.
 @node Maintaining
 @chapter Maintaining Large Programs
+@cindex maintaining large programs
+@cindex large programming projects, maintaining
 
   This chapter describes Emacs features for maintaining medium- to
 large-size programs and packages.  These features include:
diff --git a/doc/emacs/modes.texi b/doc/emacs/modes.texi
index 56b779f8de6..1a32f619478 100644
--- a/doc/emacs/modes.texi
+++ b/doc/emacs/modes.texi
@@ -127,7 +127,7 @@ see which mode is actually being entered.
 Modes}).  For example, you can put the following lines in your init
 file to enable Flyspell minor mode in all text-based major modes
 (@pxref{Spelling}), and ElDoc minor mode in Emacs Lisp mode
-(@pxref{Lisp Doc}):
+(@pxref{Programming Language Doc}):
 
 @example
 (add-hook 'text-mode-hook 'flyspell-mode)
diff --git a/doc/emacs/programs.texi b/doc/emacs/programs.texi
index b5e577d96a4..3c08909d418 100644
--- a/doc/emacs/programs.texi
+++ b/doc/emacs/programs.texi
@@ -1197,7 +1197,7 @@ use in your program.
 @menu
 * Info Lookup::         Looking up library functions and commands in Info files.
 * Man Page::            Looking up man pages of library functions and commands.
-* Lisp Doc::            Looking up Emacs Lisp functions, etc.
+* Programming Language Doc:: Looking up program functions, variables, etc.
 @end menu
 
 @node Info Lookup
@@ -1298,8 +1298,10 @@ WoMan Manual}.
 the WoMan Info manual, which is distributed with Emacs.
 @end ifnotinfo
 
-@node Lisp Doc
-@subsection Emacs Lisp Documentation Lookup
+@node Programming Language Doc
+@subsection Programming Language Documentation Lookup
+@cindex documentation for program symbols
+@cindex program functions and variables, documentation lookup
 
   When editing Emacs Lisp code, you can use the commands @kbd{C-h f}
 (@code{describe-function}) and @kbd{C-h v} (@code{describe-variable})
@@ -1307,43 +1309,120 @@ to view the built-in documentation for the Lisp functions and
 variables that you want to use.  @xref{Name Help}.
 
 @cindex ElDoc mode
+@cindex at-point documentation for program symbols
 @findex eldoc-mode
 @findex global-eldoc-mode
-  ElDoc is a buffer-local minor mode that helps with looking up Lisp
-documentation.  When it is enabled, the echo area displays some useful
-information whenever there is a Lisp function or variable at point;
-for a function, it shows the argument list, and for a variable it
-shows the first line of the variable's documentation string.  To
-toggle ElDoc mode, type @kbd{M-x eldoc-mode}.  There's also a Global
-ElDoc mode, which is turned on by default, and affects buffers whose
-major mode sets the variables described below.  Use @w{@kbd{M-x
-global-eldoc-mode}} to turn it off globally.
-
-@vindex eldoc-documentation-strategy
-@vindex eldoc-documentation-functions
-  These variables can be used to configure ElDoc mode:
+  ElDoc@footnote{
+The name ``ElDoc'' is a historical accident: this mode started by
+supporting Emacs Lisp buffers.
+} is a buffer-local minor mode that helps with looking up
+documentation of symbols (functions, methods, classes, variables,
+etc.) in your program.  When this mode is enabled, the echo area
+displays useful information whenever there is a documented symbol at
+point.  For example, in buffers under the Emacs Lisp mode, it shows
+the argument list of a function at point, and for a Lisp variable it
+shows the first line of the variable's documentation string.
+
+To toggle ElDoc mode, type @kbd{M-x eldoc-mode}.  There's also a
+Global ElDoc mode, which is turned on by default, and turns on the
+ElDoc mode in buffers whose major mode sets the variables described
+below.  Use @w{@kbd{M-x global-eldoc-mode}} to turn it off globally.
+
+Various major modes configure the Global ElDoc mode to use their
+documentation functions.  Examples include Emacs Lisp mode, Python
+mode, and Cfengine mode.  In addition, Emacs features that provide
+support for several major modes configure ElDoc to use their
+facilities for retrieving the documentation.  Examples include Eglot
+(@pxref{Eglot Features,,, eglot, Eglot: The Emacs LSP Client}), which
+provides documentation based on information from language servers;
+Semantic's Idle Summary mode (@pxref{Idle Summary Mode,,, semantic,
+Semantic Manual}); and Flymake, which uses ElDoc to show diagnostics
+at point (@pxref{Finding diagnostics,,, flymake, GNU Flymake manual}).
+
+The ElDoc mode works by scheduling the display of the available
+documentation for the symbol at point after Emacs has been idle for
+some short time.  This avoids annoying flickering of documentation
+messages in the echo area or the mode line when you type quickly and
+without delay.
+
+@findex eldoc-print-current-symbol-info
+You can also trigger the display of documentation for a symbol at
+point by using the command @kbd{M-x eldoc-print-current-symbol-info}.
+
+  The following variables can be used to configure ElDoc mode:
+
+@vtable @code
+@item eldoc-idle-delay
+The value of this user option controls the amount of idle time before
+the at-point documentation is displayed.  It should be set to the
+number of seconds to wait; the value of zero means to display without
+any delay.  the default is 0.5 sec.
+
+@item eldoc-print-after-edit
+If this user option is non-@code{nil}, ElDoc will show documentation
+only after some editing command, like inserting or deleting some
+text.  This comes in handy if you want Emacs to display documentation
+only about symbols that you type, but not about symbols that are
+already in the buffer (so just reading the source code will not show
+documentation).  The default value is @code{nil}.  If you change the
+value, you need to toggle @code{eldoc-mode} off and on again.
+
+@item eldoc-echo-area-use-multiline-p
+This user option controls whether and how to truncate documentation
+text if it is longer than the echo-area can display as a single screen
+line.  If the value is a positive number, it specifies the number of
+screen lines that ElDoc is allowed to display in the echo area without
+truncating the documentation.  A positive integer specifies the
+absolute maximum number of screen lines to use; a floating-point
+number specifies the number of screen lines as a fraction of the
+frame's height.  The value of @code{t} means never truncate the
+documentation (the echo-area will be resized up to the height allowed
+by @code{max-mini-window-height}, @pxref{Minibuffer Edit}), whereas
+the value of @code{nil} means truncate if the documentation is longer
+than a single screen line.  Finally, the special value
+@code{truncate-sym-name-if-fit} (the default) means to truncate the
+part of the documentation that represents a symbol's name if doing
+that will allow the documentation to fit on a single screen line.
+
+@item eldoc-echo-area-display-truncation-message
+If non-@code{nil} (the default), and documentation shown in the echo
+area is truncated because it's too long, follow the documentation by
+instructions about how to view the complete documentation text.  If
+@code{nil}, just indicate with @samp{@dots{}} that the documentation
+was truncated.
+
+@findex eldoc-doc-buffer
+@item eldoc-echo-area-prefer-doc-buffer
+If the value of this user option is @code{t}, ElDoc will not show the
+documentation in the echo area if the ElDoc buffer with the
+documentation is already displayed in some window.  (You can use the
+command @kbd{M-x eldoc-doc-buffer} any time to show the ElDoc buffer.)
+If the value of this option is the symbol @code{maybe}, the
+documentation will not be displayed in the echo area if the ElDoc
+buffer is shown in some window and the documentation text has to be
+truncated if displayed in the echo area.  Finally, the value of
+@code{nil} (the default) means always show the documentation in the
+echo area.
 
-@table @code
 @item eldoc-documentation-strategy
-This variable holds the function which is used to retrieve
-documentation for the item at point from the functions in the hook
-@code{eldoc-documentation-functions}.  By default,
-@code{eldoc-documentation-strategy} returns the first documentation
-string produced by the @code{eldoc-documentation-functions} hook, but
-it may be customized to compose those functions' results in other
-ways.
+This customizable variable's value is the function which is used to
+retrieve and display documentation for the symbol at point.  The
+documentation is produced by the functions in the hook
+@code{eldoc-documentation-functions}.  The default value of
+@code{eldoc-documentation-strategy} specifies that ElDoc should
+display the first documentation text produced by functions in the
+@code{eldoc-documentation-functions} hook, but you can customize
+@code{eldoc-documentation-strategy} to work in other ways, such as
+displaying all of the documentation texts concatenated together.
 
 @item eldoc-documentation-functions
-This abnormal hook holds documentation functions.  It acts as a
-collection of backends for ElDoc.  This is what modes should use to
-register their documentation functions with ElDoc.
-
-@vindex eldoc-echo-area-display-truncation-message
-@item eldoc-echo-area-display-truncation-message
-If non-@code{nil} (the default), display a verbose message about how
-to view a complete documentation (if it has been truncated in the echo
-area).  If @code{nil}, just mark truncated messages with @samp{...}.
-@end table
+This abnormal hook's value is a list of functions that can produce
+documentation for the symbol at point as appropriate for the current
+buffer's major-mode.  These functions act as a collection of backends
+for ElDoc.  Major mode register their documentation lookup functions
+with ElDoc by adding their functions to the buffer-local value of this
+variable.
+@end vtable
 
 @node Hideshow
 @section Hideshow minor mode
diff --git a/doc/lispref/symbols.texi b/doc/lispref/symbols.texi
index ea1e086ebf1..1074a8a8292 100644
--- a/doc/lispref/symbols.texi
+++ b/doc/lispref/symbols.texi
@@ -718,9 +718,9 @@ byte-compiling any of the two files has equivalent results.  The
 shorthands @code{snu-split} and @code{snu-lines} used in the second
 version are @emph{not} interned in the obarray.  This is easily seen
 by moving point to the location where the shorthands are used and
-waiting for ElDoc (@pxref{Lisp Doc, , Local Variables in Files, emacs,
-The GNU Emacs Manual}) to hint at the true full name of the symbol
-under point in the echo area.
+waiting for ElDoc (@pxref{Programming Language Doc, , Local Variables
+in Files, emacs, The GNU Emacs Manual}) to hint at the true full name
+of the symbol under point in the echo area.
 
 Since @code{read-symbol-shorthands} is a file-local variable, it is
 possible that multiple libraries depending on
diff --git a/doc/misc/octave-mode.texi b/doc/misc/octave-mode.texi
index 31d64c3d840..d7ea54198ad 100644
--- a/doc/misc/octave-mode.texi
+++ b/doc/misc/octave-mode.texi
@@ -204,9 +204,10 @@ The GNU Emacs Manual}).  Currently, function names can be indexed.
 
 @cindex ElDoc Mode Support
 @vindex octave-eldoc-message-style
-ElDoc mode (@pxref{Lisp Doc,,, emacs, The GNU Emacs Manual}) is
-supported.  By customizing @code{octave-eldoc-message-style} it can be
-changed from displaying one or multi line hints.
+ElDoc mode (@pxref{Programming Language Doc,,, emacs, The GNU Emacs
+Manual}) is supported.  By customizing
+@code{octave-eldoc-message-style} it can be changed from displaying
+one or multi line hints.
 
 @c @cindex TAGS
 @c @cindex Emacs TAGS files
diff --git a/doc/misc/sem-user.texi b/doc/misc/sem-user.texi
index 3141ab7c692..36e09b152e5 100644
--- a/doc/misc/sem-user.texi
+++ b/doc/misc/sem-user.texi
@@ -621,7 +621,7 @@ Run the Semantic idle work function with debugging turned on.
 Semantic Idle Summary mode is a minor mode that displays a short
 summary of the symbol at point, such as its function prototype, in the
 echo area.  Its functionality is similar to what ElDoc mode provides
-for Emacs Lisp (@pxref{Lisp Doc,,,emacs,Emacs manual}).
+for Emacs Lisp (@pxref{Programming Language Doc,,,emacs,Emacs manual}).
 
 @deffn global-semantic-idle-summary-mode &optional arg
 This command toggles Semantic Idle Summary mode in all
diff --git a/lisp/emacs-lisp/eldoc.el b/lisp/emacs-lisp/eldoc.el
index 6fd89a690dc..e1801c45b75 100644
--- a/lisp/emacs-lisp/eldoc.el
+++ b/lisp/emacs-lisp/eldoc.el
@@ -55,21 +55,24 @@
   :group 'extensions)
 
 (defcustom eldoc-idle-delay 0.50
-  "Number of seconds of idle time to wait before printing.
+  "Number of seconds of idle time to wait before displaying documentation.
 If user input arrives before this interval of time has elapsed after the
-last input, no documentation will be printed.
+last input event, no documentation will be displayed.
 
-If this variable is set to 0, no idle time is required."
+If this variable is set to 0, display the documentation without any delay."
   :type 'number)
 
 (defcustom eldoc-print-after-edit nil
-  "If non-nil, eldoc info is only shown when editing.
+  "If non-nil, eldoc info is only shown after editing commands.
 Changing the value requires toggling `eldoc-mode'."
   :type 'boolean)
 
 (defcustom eldoc-echo-area-display-truncation-message t
   "If non-nil, provide verbose help when a message has been truncated.
-If nil, truncated messages will just have \"...\" appended."
+When this is non-nil, and the documentation string was truncated to
+fit in the echo-area, the documentation will be followed by an
+explanation of how to display the full documentation text.
+If nil, truncated messages will just have \"...\" to indicate truncation."
   :type 'boolean
   :version "28.1")
 
@@ -93,22 +96,24 @@ Note that this variable has no effect, unless
 
 (defcustom eldoc-echo-area-use-multiline-p 'truncate-sym-name-if-fit
   "Allow long ElDoc doc strings to resize echo area display.
-If value is t, never attempt to truncate messages, even if the
-echo area must be resized to fit.
+If the value is t, never attempt to truncate messages, even if the
+echo area must be resized to fit.  In that case, Emacs will resize
+the mini-window up to the limit set by `max-mini-window-height'.
 
 If the value is a positive number, it is used to calculate a
-number of logical lines of documentation that ElDoc is allowed to
-put in the echo area.  If a positive integer, the number is used
-directly, while a float specifies the number of lines as a
-proportion of the echo area frame's height.
+number of screen lines of documentation that ElDoc is allowed to
+put in the echo area.  A positive integer specifies the maximum
+number of lines directly, while a floating-point number specifies
+the number of screen lines as a fraction of the echo area frame's
+height.
 
-If value is the symbol `truncate-sym-name-if-fit', the part of
+If the value is the symbol `truncate-sym-name-if-fit', the part of
 the doc string that represents a symbol's name may be truncated
 if it will enable the rest of the doc string to fit on a single
 line, without resizing the echo area.
 
-If value is nil, a doc string is always truncated to fit in a
-single line of display in the echo area.
+If the value is nil, a doc string is always truncated to fit in a
+single screen line of echo-area display.
 
 Any resizing of the echo area additionally respects
 `max-mini-window-height'."
@@ -121,12 +126,12 @@ Any resizing of the echo area additionally respects
  line" truncate-sym-name-if-fit)))
 
 (defcustom eldoc-echo-area-prefer-doc-buffer nil
-  "Prefer ElDoc's documentation buffer if it is showing in some frame.
+  "Prefer ElDoc's documentation buffer if it is displayed in some window.
 If this variable's value is t, ElDoc will skip showing
 documentation in the echo area if the dedicated documentation
-buffer (given by `eldoc-doc-buffer') is being displayed in some
-window.  If the value is the symbol `maybe', then the echo area
-is only skipped if the documentation doesn't fit there."
+buffer (displayed by `eldoc-doc-buffer') is already displayed in
+some window.  If the value is the symbol `maybe', then the echo area
+is only skipped if the documentation needs to be truncated there."
   :type 'boolean)
 
 (defface eldoc-highlight-function-argument
@@ -287,8 +292,10 @@ reflect the change."
 (put 'eldoc-mode-line-string 'risky-local-variable t)
 
 (defun eldoc-minibuffer-message (format-string &rest args)
-  "Display messages in the mode-line when in the minibuffer.
-Otherwise work like `message'."
+  "Display message specified by FORMAT-STRING and ARGS on the mode-line as needed.
+This function displays the message produced by formatting ARGS
+with FORMAT-STRING on the mode line when the current buffer is a minibuffer.
+Otherwise, it displays the message like `message' would."
   (if (minibufferp)
       (progn
 	(add-hook 'minibuffer-exit-hook
@@ -632,8 +639,8 @@ If INTERACTIVE is t, also display the buffer."
   (when interactive (eldoc-doc-buffer t)))
 
 (defun eldoc-documentation-default ()
-  "Show first doc string for item at point.
-Default value for `eldoc-documentation-strategy'."
+  "Show the first non-nil documentation string for item at point.
+This is the default value for `eldoc-documentation-strategy'."
   (run-hook-with-args-until-success 'eldoc-documentation-functions
                                     (eldoc--make-callback :patient)))
 
@@ -651,18 +658,18 @@ else wait for all doc strings."
   t)
 
 (defun eldoc-documentation-compose ()
-  "Show multiple doc strings at once after waiting for all.
-Meant as a value for `eldoc-documentation-strategy'."
+  "Show multiple documentation strings together after waiting for all of them.
+This is meant to be used as a value for `eldoc-documentation-strategy'."
   (eldoc--documentation-compose-1 nil))
 
 (defun eldoc-documentation-compose-eagerly ()
-  "Show multiple doc strings at once as soon as possible.
-Meant as a value for `eldoc-documentation-strategy'."
+  "Show multiple documentation strings one by one as soon as possible.
+This is meant to be used as a value for `eldoc-documentation-strategy'."
   (eldoc--documentation-compose-1 t))
 
 (defun eldoc-documentation-enthusiast ()
-  "Show most important doc string produced so far.
-Meant as a value for `eldoc-documentation-strategy'."
+  "Show most important documentation string produced so far.
+This is meant to be used as a value for `eldoc-documentation-strategy'."
   (run-hook-wrapped 'eldoc-documentation-functions
                     (lambda (f)
                       (let* ((callback (eldoc--make-callback :enthusiast))
@@ -692,40 +699,42 @@ Meant as a value for `eldoc-documentation-strategy'."
 (eldoc--documentation-strategy-defcustom eldoc-documentation-strategy
     eldoc-documentation-function
   #'eldoc-documentation-default
-  "How to collect and organize results of `eldoc-documentation-functions'.
-
-This variable controls how `eldoc-documentation-functions', which
-specifies the sources of documentation, is queried and how its
-results are organized before being displayed to the user.  The
-following values are allowed:
-
-- `eldoc-documentation-default': calls functions in the special
-  hook in order until one is found that produces a doc string
-  value.  Display only that value;
-
-- `eldoc-documentation-compose': calls all functions in the
-  special hook and displays all of the resulting doc strings
-  together.  Wait for all strings to be ready, and preserve their
-  relative order as specified by the order of functions in the hook;
-
-- `eldoc-documentation-compose-eagerly': calls all functions in
-  the special hook and displays as many of the resulting doc
-  strings as possible, as soon as possible.  Preserves the
-  relative order of doc strings;
-
-- `eldoc-documentation-enthusiast': calls all functions in the
-  special hook and displays only the most important resulting
-  docstring one at any given time.  A function appearing first in
-  the special hook is considered more important.
-
-This variable can also be set to a function of no args that
-returns something other than a string or nil and allows for some
+  "How to collect and display results of `eldoc-documentation-functions'.
+
+This variable controls how to call the functions in the special hook
+`eldoc-documentation-functions', and how to organize their results
+for display to the user.  The functions in `eldoc-documentation-functions'
+are the source of documentation, and act as back-end for ElDoc.
+
+The following values are supported:
+
+- `eldoc-documentation-default': Call functions in the special
+  hook in order, until one of them returns a non-nil string
+  value.  Display only that string.
+
+- `eldoc-documentation-compose': Call all the functions in the
+  special hook and display all of the resulting strings together,
+  after all of the functions were called, and in the order of the
+  functions in the hook.
+
+- `eldoc-documentation-compose-eagerly': Call all the functions in
+  the special hook, and display each non-nil string as soon as it
+  is returned by a function, before calling the next function.
+
+- `eldoc-documentation-enthusiast': Call all the functions in the
+  special hook, and display only the most important resulting
+  string at any given time.  A function appearing first in
+  the special hook is considered more important than those which
+  appear after it.
+
+This variable can also be set to a function of no arguments that
+returns something other than a string or nil, and allows for some
 or all of the special hook `eldoc-documentation-functions' to be
 run.  In that case, the strategy function should follow that
-other variable's protocol closely and endeavor to display the
-resulting doc strings itself.
+other variable's protocol closely and display the resulting doc
+strings itself.
 
-For backward compatibility to the \"old\" protocol, this variable
+For backward compatibility with the \"old\" protocol, this variable
 can also be set to a function that returns nil or a doc string,
 depending whether or not there is documentation to display at
 all."