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

[Eli Zaretskii <eliz@gnu.org>]

> From: Sebastian Urban <mrsebastianurban@gmail.com>
> Cc: 36365@debbugs.gnu.org
> Date: Fri, 2 Aug 2019 12:52:23 +0200
> 
> > I don't think there's anything wrong with the documentation you
> > cite, and I thought I already explained why I thought so in this
> > discussion.
> 
> Well, now I'm confused.  I reread the thread and I found nothing but
> "I see nothing wrong with the doc string as it is" as an explanation
> for not changing the doc string of 'mouse-start-secondary'.  And I'm
> still going to write that it's missing information about deselecting,
> which could be useful.

We've established that it isn't possible to deselect without making a
new selection.  So I don't think deselection is something we can
describe, as it doesn't really exist.

> As for description of 'M-mouse-1' in Emacs Manual, I will also write
> that the "use @kbd{M-mouse-3} to set the other end and complete the
> selection" part of the sentence below is unnecessary.
> 
>  From KILLING.TEXI (L604-610):
>   @findex mouse-start-secondary
>   @kindex M-mouse-1
>   @item M-mouse-1
>   Set one endpoint for the @dfn{secondary selection}
>   (@code{mouse-start-secondary}); use @kbd{M-mouse-3} to set the other
>   end and complete the selection.  This command cancels any existing
>   secondary selection, when it starts a new one.
> 
> But in this case, if you have a reason for this, that I cannot see,
> I'll leave this one alone.

Yes, the manual should generally present a coherent text, so the
addition looked like an improvement to me.