Explain better what the interactive prefix does in scroll-down/up

* lisp/window.el (scroll-up-command, scroll-down-command):
* lisp/image-mode.el (image-scroll-up, image-scroll-down):
Actually explain what the interactive prefix does (bug#44503).

2 files changed, 30 insertions, 6 deletions

diff --git a/lisp/image-mode.el b/lisp/image-mode.el
index 721f2f2bbd8..ea5d7ff0f35 100644
--- a/lisp/image-mode.el
+++ b/lisp/image-mode.el
@@ -282,10 +282,17 @@ Stop if the top edge of the image is reached."
 (defun image-scroll-up (&optional n)
   "Scroll image in current window upward by N lines.
 Stop if the bottom edge of the image is reached.
-If ARG is omitted or nil, scroll upward by a near full screen.
+
+Interactively, giving this command a numerical prefix will scroll
+up by that many lines (and down by that many lines if the number
+is negative).  Without a prefix, scroll up by a full screen.
+If given a `C-u -' prefix, scroll a full page down instead.
+
+If N is omitted or nil, scroll upward by a near full screen.
 A near full screen is `next-screen-context-lines' less than a full screen.
-Negative ARG means scroll downward.
-If ARG is the atom `-', scroll downward by nearly full screen.
+A negative N means scroll downward.
+
+If N is the atom `-', scroll downward by nearly full screen.
 When calling from a program, supply as argument a number, nil, or `-'."
   (interactive "P")
   (cond ((null n)
@@ -303,10 +310,17 @@ When calling from a program, supply as argument a number, nil, or `-'."
 (defun image-scroll-down (&optional n)
   "Scroll image in current window downward by N lines.
 Stop if the top edge of the image is reached.
-If ARG is omitted or nil, scroll downward by a near full screen.
+
+Interactively, giving this command a numerical prefix will scroll
+down by that many lines (and up by that many lines if the number
+is negative).  Without a prefix, scroll down by a full screen.
+If given a `C-u -' prefix, scroll a full page up instead.
+
+If N is omitted or nil, scroll downward by a near full screen.
 A near full screen is `next-screen-context-lines' less than a full screen.
-Negative ARG means scroll upward.
-If ARG is the atom `-', scroll upward by nearly full screen.
+A negative N means scroll upward.
+
+If N is the atom `-', scroll upward by nearly full screen.
 When calling from a program, supply as argument a number, nil, or `-'."
   (interactive "P")
   (cond ((null n)
diff --git a/lisp/window.el b/lisp/window.el
index 9f787846124..52003f7b7b4 100644
--- a/lisp/window.el
+++ b/lisp/window.el
@@ -10031,6 +10031,11 @@ When point is already on that position, then signal an error."
 
 (defun scroll-up-command (&optional arg)
   "Scroll text of selected window upward ARG lines; or near full screen if no ARG.
+Interactively, giving this command a numerical prefix will scroll
+up by that many lines (and down by that many lines if the number
+is negative).  Without a prefix, scroll up by a full screen.
+If given a `C-u -' prefix, scroll a full page down instead.
+
 If `scroll-error-top-bottom' is non-nil and `scroll-up' cannot
 scroll window further, move cursor to the bottom line.
 When point is already on that position, then signal an error.
@@ -10063,6 +10068,11 @@ If ARG is the atom `-', scroll downward by nearly full screen."
 
 (defun scroll-down-command (&optional arg)
   "Scroll text of selected window down ARG lines; or near full screen if no ARG.
+Interactively, giving this command a numerical prefix will scroll
+down by that many lines (and up by that many lines if the number
+is negative).  Without a prefix, scroll down by a full screen.
+If given a `C-u -' prefix, scroll a full page up instead.
+
 If `scroll-error-top-bottom' is non-nil and `scroll-down' cannot
 scroll window further, move cursor to the top line.
 When point is already on that position, then signal an error.