bug#31858: 27.0; doc of `window-toggle-side-windows'

[Eli Zaretskii]

> Date: Sat, 16 Jun 2018 21:20:44 -0700 (PDT)
> From: Drew Adams <drew.adams@oracle.com>
> Cc: npostavs@gmail.com, 31858@debbugs.gnu.org
> 
> > Would replacing this:
> >   Toggle side windows on specified FRAME.
> > with this:
> >   Toggle display of side windows on specified FRAME.
> > solve your problem with this doc string?
> 
> It's definitely an improvement.

OK, I did that, and also changed the wording to be a bit more
reader-friendly.