Further improvements to completion-list-mode-map

Further improvements to completion-list-mode-map

* doc/emacs/mini.texi (Completion Commands): Mention the change,
and mention the 'n' and 'p' keys bound to 'next-completion' and
'previous-completion'. (bug#47699).
* lisp/minibuffer.el (minibuffer-local-completion-map): Change the
M-g key to M-g M-c.

* lisp/simple.el (completion-list-mode-map): Change the M-g key to
M-g M-c.
(read-expression-map): Bind M-g M-c to
read-expression-switch-to-completions.
(read-expression-switch-to-completions): New function.