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

[Drew Adams]

Following up on the request by Nicolas Goaziou in bug #28263, please
consider adding some information about the keys that users can bind.

Please:

1. Say that Emacs and 3rd-party Lisp libraries often bind keys, but that
   some keys are specifically reserved, by convention, for users.  Point
   out which keys are reserved for users.

2. Make it clear that users can, in any case, bind ANY keys they want;
   they are not limited to binding user-reserved keys.  In particular,
   users can rebind keys that Emacs or some 3rd-party library has already
   bound.

3. State that after a user has bound a key, evaluating some Emacs code
   (including loading a Lisp library) might rebind that key if it is not
   reserved for users.  This is the reason some keys are reserved for
   users: to prevent the bother of some non-user code overriding user
   key bindings.

The keys reserved for users are currently specified only in the Elisp
manual (node `Key Binding Conventions').  Please consider adding a link
to that node from wherever the user-reserved keys will be called out in
the Emacs manual.

A likely location in the Emacs manual for such info would be somewhere
under node `Customizing Key Bindings'.

In GNU Emacs 26.0.91 (build 1, x86_64-w64-mingw32)
 of 2018-01-22
Repository revision: 752fba992b793a74d202c9cfc3e1a92fd458e748
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''