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

[積丹尼 Dan Jacobson]

>>>>> "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

Well then just add the bindings. Thanks.

Apparently if users add them on their own it will mess things up.