bug#39847: Document how users can make text-zoom keys same as browser

[Drew Adams]

> I also wished that C-+ and C-- would scale
> the screen, just a few months ago.
> 
> Perhaps we don't need C-- to make the argument negative.  Perhaps
> it is enough that M-- does that.

My preference would be against doing that.
It's true that `M--' does the same thing -
so we may not _need_ `C--'.  But Control
is very handy for prefix args.

---

There are lots of things that users are
used to with browsers, which conflict with
Emacs bindings.

Wrt zooming, my guess is that more users
use a mouse wheel to zoom in a browser (and
in other applications) than use C-(+/-).
I use that (mouse wheel) all the time in
Emacs to zoom.  For me it's handier than
`C-x +/-'.  (But different people prefer
different things.)