bug#48779: [External] : bug#48779: (:documentation mydoc) not working

[Drew Adams]

> This undocumented feature only works in lexical-binding buffers.

My questions here would be:

1. Why is it undocumented?
2. Why does it work only for lexical-binding buffers?

___

:documentation is documented in the Elisp manual,
but only for `cl-defgeneric'.

(Grepping the Lisp source code shows there are :doc,
:doc-spec, and :documentation.  :doc is documented
in the Elisp manual, in the context of defcustom.
:doc-spec seems to be documented in no manual.)

We don't document everything, of course.  But should
:documentation be documented for defun, as well as
for cl-defgeneric?

Yes, the latter is an emulation of Common Lisp's
`defgeneric', which has :documentation.  But defun
also has it now, apparently.