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

[Eli Zaretskii]

> From: Howard Melman <hmelman@gmail.com>
> Date: Fri, 31 Jan 2020 12:23:45 -0500
> Cc: 39295@debbugs.gnu.org
> 
> > Not every feature is documented in the manual, only the important ones
> > are.  And I'm not sure I see the utility of stating in comments that
> > we don't document this in the manual: we don't say that about all the
> > other features not mentioned in the manual.
> 
> I think some place should describe when code should use kbd-help as opposed 
> to help-echo. 

I see it explained in the doc string of help-at-pt-display-when-idle,
and also in some other doc strings in help-at-pt.el.

> The elisp manual section "Properties with Special Meanings" is incomplete 
> without it.

It doesn't try to be complete.  Eventually, one needs to read the doc
strings of the features one wants to use.