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

[Sebastian Urban]

> 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.

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.