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

[Eli Zaretskii]

> From: 積丹尼 Dan Jacobson <jidanni@jidanni.org>
> Cc: 39847@debbugs.gnu.org
> Date: Sun, 01 Mar 2020 13:23:52 +0800
> 
> >>>>> "EZ" == Eli Zaretskii <eliz@gnu.org> writes:
> EZ> We don't explain the rationale for key bindings in the manual, and I
> EZ> don't think we should start doing that now.
> 
> See also #28182 .
> http://emacs-fu.blogspot.com/2008/12/zooming-inout.html
> https://stackoverflow.com/questions/5533110/emacs-zoom-in-zoom-out

Those are very old.

> Well then just add the bindings. Thanks.

The Ctrl-mwheel bindings are already provided.