* lisp/register.el (increment-register): Route it to `append-to-register',

* lisp/register.el (increment-register): Route it to `append-to-register',
if register contains text so that `C-x r +' can now be
used for appending to a text register.
(register): New group.
(register-separator): New user option.
(append-to-register, prepend-to-register): Add separator based on
`register-separator.
* doc/emacs/regs.texi (Text Registers): `C-x r +' can now be used instead of
M-x append-to-register.  New option `register-separator'.
(Number Registers): Mention that `C-x r +' is polymorphic.

Fixes: debbugs:12217