Improve completion-list-mode-map

Improve completion-list-mode-map

* doc/emacs/mini.texi (Completion Commands): Mention it.

* lisp/minibuffer.el (minibuffer-local-completion-map): Add the M-g key
for switch-to-completion (bug#47699).

* lisp/simple.el (completion-list-mode-map): Make special-mode-map its
parent, unbind the 'g' revert key, add the 'n' and 'p' keys for
next-completion and previous-completion, and the M-g key for
switch-to-minibuffer.
(switch-to-minibuffer): New function.