bug#36365: Add info how to deselect secondary selection (Emacs manual)

[Eli Zaretskii <eliz@gnu.org>]

> From: Sebastian Urban <mrsebastianurban@gmail.com>
> Date: Mon, 24 Jun 2019 19:59:42 +0200
> 
> Because, if it's "selection", people may think that to deselect all
> you need is to click 'mouse-1' somewhere, but it doesn't work, as well
> as 'C-g' and it may be confusing.
> 
> Therefore please add information about 'M-mouse-1' as a way to
> deselect, perhaps somewhere next to the L626-627 or after the
> definition of 'M-mouse-1' L607-608 in KILLING.TEXI.
> 
> Then update documentation string for 'mouse-start-secondary' with the
> same information.
> 
> ===
> 
> Also (because it's the same chapter) in Emacs manual (INFO) "12.3.3
> Secondary Selection" description of 'M-mouse-3' could be changed:
> 
> FROM
> Set the secondary selection, with one end at the position clicked
> and the other at the position specified with ‘M-mouse-1’
> (‘mouse-secondary-save-then-kill’).  This also puts the selected
> text in the kill ring.  A second ‘M-mouse-3’ at the same place
> kills the secondary selection just made.
> 
> TO
> Set the secondary selection, with one end at the position clicked and
> the other at the position specified with ‘M-mouse-1’, and put the
> selected text in the kill ring (‘mouse-secondary-save-then-kill’).
> A second ‘M-mouse-3’ at the same place kills the secondary selection
> just made.
> 
> Placement of 'mouse-secondary-save-then-kill' will be better, right
> now it may confuse people because it stands next to 'M-mouse-1' which
> has different command.

Thanks, I fixed these two issues.