Enable keys M-down, M-up, M-RET for in-buffer completion

Enable keys M-down, M-up, M-RET for in-buffer completion

* lisp/minibuffer.el (completion-in-region-mode-map): Add keybindings M-<up>
for minibuffer-previous-completion, M-<down> for minibuffer-next-completion,
M-RET for minibuffer-choose-completion.
(completion-in-region-mode): Set buffer-local
'minibuffer-completion-auto-choose' to nil.
(minibuffer-next-completion): Get the value of
'minibuffer-completion-auto-choose' from the minibuffer.
(minibuffer-previous-completion): Simplify by delegating to
'minibuffer-next-completion'.

* doc/emacs/programs.texi (Symbol Completion): Add description of keys
M-down, M-up, M-RET.

https://lists.gnu.org/archive/html/emacs-devel/2022-05/msg00916.html