Mention minibuffer-next-completion in completion-help

Mention minibuffer-next-completion in completion-help

Now that it's not necessary to switch to the *Completions* buffer to
switch between and choose completions, we should make that clear in
the help text.  Let's also make the bindings uniform both in and
outside the *Completions* buffer so users don't have to think about
the difference.

In working with new users, they've assumed that since the help text in
*Completions* says "Click on a completion to select it", that that's
the only way to select a completion.  This text should help clarify
that.

* doc/emacs/mini.texi (Completion Commands): Document new bindings.
* lisp/simple.el (completion-list-mode-map): Bind
minibuffer-{next,previous,choose}-completion.
(completion-setup-function): Mention bindings for
minibuffer-{next,previous,choose}-completion. (Bug#64425)