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

[Eli Zaretskii]

> Date: Sat, 06 Jul 2019 21:21:21 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 36365@debbugs.gnu.org
> 
> > I guessing that clicking 'M-mouse-1' anywhere in the text will cancel
> > selection, but also start new one, and doing so on mode line/header
> > line, will not start new selection, is it correct so far?  But then
> > when I do so I get:
> > <mode-line> <M-mouse-1> is undefined
> > OR
> > <header-line> <M-mouse-1> is undefined
> > and secondary selection is still there.  Am I doing something wrong?
> 
> I have no idea, as on my system secondary selections simply don't
> work.  I guess someone else will have to find the way of deselecting
> without starting a new selection, if one exists.  And if it doesn't
> exist, I will simply remove that text, so that docs match the reality.

I see now that there's no way of canceling a secondary selection
without starting a new one.  So I changed the text to reflect that.