bug#30530: 26.0; Emacs manual: mention (1) user-reserved keys, (2) users can bind any keys

[Eli Zaretskii]

> Date: Sat, 24 Feb 2018 09:00:51 -0800 (PST)
> From: Drew Adams <drew.adams@oracle.com>
> Cc: 30530@debbugs.gnu.org
> 
> The problem is that users do not necessarily have a good idea
> what keys they can bind (answer: any keys) and which keys
> Lisp libraries are likely to bind (answer: keys not reserved 
> or users).  Users have been known to ask about such things.

I added some text about the latter, although I seriously doubt that it
will be discoverable enough to make any difference.

As for the former, the manual is full of examples using all kinds of
keys in bindings (only one of them is from the reserved area, btw), so
it should be pretty clear that there are limitations on that.