In C-h k <mouse-n>, alert user to existence of any matching down-mouse-event

In C-h k <mouse-n>, alert user to existence of any matching down-mouse-event

, and instruct her to hold the mouse button to display its documentation.

* lisp/help.el (help-downify-mouse-event-type): New function.
(help-read-key-sequence, describe-key): handle double-click-time being nil or
t.
(describe-key): Print out instructions for displaying documentation of
matching mouse down key sequence command when such exists.