bug#30516: 26.0; `C-h k' for mouse click is now confusing and less useful

[Drew Adams]

A change for the worse seems to have been made to the help displayed
by `C-h k' for a mouse click.

Instead of clearly showing two separate help sections, one for the up
event and one for the down event, what you see now is help for the up
event followed immediately - with no separation, by this sentence:

  For documentation of the corresponding mouse down
  event <down-mouse-1>, click and hold the mouse button
  longer than 0.5 second(s).

That sentence follows text that describes the _command_ that is bound
to the up event.  But that sentence is NOT about that command.  It is
instead about an entirely different key (event): the down event.

This can only confuse users.  It is especially pernicious because that
sentence can be misread as being about the context of the command bound
to the up event.

Please clearly separate this information about `C-h k' from information
about the up-even command.  And consider putting it in parentheses.

And please consider having a link in *Help* that shows you the help for
the down event, instead of making a user go back to the original buffer
and repeat `C-h k' holding the mouse-button pressed > 0.5 sec.  Making a
user do that makes little (no?) sense.  That down-event information
should be available when the up-event is described.  Just show a link
that lets users see that down-event information.

This help was clearer before, it seems to me.  If there was a real
need for some kind of change, please accommodate that need with
more sensible and less confusing help information.  It should be
possible to do better than what we see now.  Thx.

In GNU Emacs 26.0.91 (build 1, x86_64-w64-mingw32)
 of 2018-01-22
Repository revision: 752fba992b793a74d202c9cfc3e1a92fd458e748
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''