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

[Howard Melman]

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. 

In GNU Emacs 26.3 (build 1, x86_64-apple-darwin18.2.0, NS appkit-1671.20 
Version 10.14.3 (Build 18D109))
of 2019-09-02 built on builder10-14.porkrind.org

-- 

Howard