bug#39295: 26.3; kbd-help property is undocumented

[Eli Zaretskii]

> From: Howard Melman <hmelman@gmail.com>
> Date: Sun, 26 Jan 2020 12:03:12 -0500
> 
> The docstring for `help-at-pt-string' mentions a `kbd-help'
> property, but this property is not mentioned in either the
> emacs or elisp manuals.
> 
> The commentary to help-at-pt.el mentions that it is a new
> property though it's not clear how new and it doesn't seem
> to be used in the rest of emacs 26 lisp source. 

It indeed is unused, so I'm not sure it should be documented at this
time.  As for "new": the help-at-pt.el file was added to Emacs 17
years ago, and its original version already included support for this
property.