OClosure: New function `function-documentation`

As mentioned in the original OClosure commit, OClosures (ab)use the
bytecode's docstring slot to hold the OClosure's type.  This currently
prevents OClosures from having their own docstring.

Introduce a new generic function `function-documentation` to fetch the
docstring of a function, which can then be implemented in various
different ways depending on the OClosure's type.

* lisp/simple.el (function-documentation): New generic function.
(bad-package-check): Strength-reduce `eval` to `symbol-value`.
* src/doc.c (Fdocumentation): Use it.

* lisp/emacs-lisp/oclosure.el (oclosure--accessor-docstring): New function.
* test/lisp/emacs-lisp/oclosure-tests.el (oclosure-test):
Add test for accessor's docstrings.

1 files changed, 7 insertions, 0 deletions

diff --git a/doc/lispref/help.texi b/doc/lispref/help.texi
index 10a12940a15..d53bfad8e9e 100644
--- a/doc/lispref/help.texi
+++ b/doc/lispref/help.texi
@@ -158,6 +158,13 @@ the function definition has no documentation string.  In that case,
 @code{documentation} returns @code{nil}.
 @end defun
 
+@defun function-documentation function
+Generic function used by @code{documentation} to extract the raw
+docstring from a function object.  You can specify how to get the
+docstring of a specific function type by adding a corresponding method
+to it.
+@end defun
+
 @defun face-documentation face
 This function returns the documentation string of @var{face} as a
 face.