bug#58602: 29.0.50; Please document (:documentation FORM) spec for closures

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

I pushed the text below.
Thanks to Eli and Drew for their review.


        Stefan


PS: BTW, I noticed that `i :documentation RET` doesn't bring me to this
text, instead it just looks for "documentation" (which matches many
more index entries), disregarding the `:` and making this `@kindex` much
less useful.

PPS: Every time you use this `function-documentation` property, kitten
suffer atrocious pains.


diff --git a/doc/lispref/functions.texi b/doc/lispref/functions.texi
index 8b858e0aa01..94c42ba260d 100644
--- a/doc/lispref/functions.texi
+++ b/doc/lispref/functions.texi
@@ -533,6 +533,46 @@ Function Documentation
 compiler emit a warning message when it compiles Lisp programs which
 use the deprecated calling convention.
 
+@cindex computed documentation string
+@kindex{:documentation}
+
+Documentation strings are usually static, but occasionally it can be
+necessary to generate them dynamically.  In some cases you can do so
+by writing a macro which generates at compile time the code of the
+function, including the desired documentation string.  But you can
+also generate the docstring dynamically by writing
+@code{(:documentation @var{form})} instead of the documentation
+string.  This will evaluate @var{form} at run-time when the function
+is defined and use it as the documentation string@footnote{This only
+works in code using @code{lexical-binding}.}.  You can also compute
+the documentation string on the fly when it is requested, by setting
+the @code{function-documentation} property of the function's symbol to
+a Lisp form that evaluates to a string.
+
+For example:
+@example
+@group
+(defun adder (x)
+  (lambda (y)
+    (:documentation (format "Add %S to the argument Y." x))
+    (+ x y)))
+(defalias 'adder5 (adder 5))
+(documentation 'adder5)
+    @result{} "Add 5 to the argument Y."
+@end group
+
+@group
+(put 'adder5 'function-documentation
+     '(concat (documentation (symbol-function 'adder5) 'raw)
+              "  Consulted at " (format-time-string "%H:%M:%S")))
+(documentation 'adder5)
+    @result{} "Add 5 to the argument Y.  Consulted at 15:52:13"
+(documentation 'adder5)
+    @result{} "Add 5 to the argument Y.  Consulted at 15:52:18"
+@end group
+@end example
+
+
 @node Function Names
 @section Naming a Function
 @cindex function definition