Optionally avoid extending 'completion-at-point-functions'

Optionally avoid extending 'completion-at-point-functions'

It is now possible to avoid extending
'completion-at-point-functions' in Text mode and its descendants.
* lisp/textmodes/text-mode.el
(text-mode-meta-tab-ispell-complete-word): Rename to...
(text-mode-ispell-word-completion): ...this.  Extend with another
option 'completion-at-point'.
(text-mode): Only extend 'completion-at-point-functions' when
'text-mode-ispell-word-completion' is 'completion-at-point'.
(Bug#67527)

* etc/NEWS: Update the entry about 'M-TAB' in Text mode.