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

[Stefan Kangas]

(Sending a copy also to Bug#28182 where this has been previously
discussed.)

Drew Adams <drew.adams@oracle.com> writes:

>> 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.

I suggest binding this to s-- and s-+.  Would that be an acceptable
compromise?  It would be backwards compatible, and these keys are
currently unbound.

Best regards,
Stefan Kangas