bug#67527: 30.0.50; FR: Add an option to disable ispell completion in text-mode

[Eli Zaretskii <eliz@gnu.org>]

> From: Daniel Mendler <mail@daniel-mendler.de>
> Cc: Eason Huang <aqua0210@foxmail.com>,  Eshel Yaron <me@eshelyaron.com>,
>   67527@debbugs.gnu.org,  simon@ecksd.com
> Date: Sat, 20 Jan 2024 10:46:00 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Eshel, given this feedback, I think it would make sense to add a new
> > defcustom that would make text-mode avoid adding
> > ispell-completion-at-point to completion-at-point-functions.  Would
> > you please submit such a change and the documentation changes to go
> > with it?  TIA.
> 
> Maybe the existing defcustom `text-mode-meta-tab-ispell-complete-word'
> could be renamed and reused, taking three values? This way we avoid
> introducing multiple defcustoms for tweaking the same feature.
> 
> - `command': Bind `ispell-complete-word' to TAB.
> - `capf': Add `ispell-completion-at-point' to the
>   `completion-at-point-functions'.
> - `nil': None of the above.

Reused, yes.  Renamed, I'd rather prefer we avoided that, as it causes
compatibility problems.  If we can extend its list of valid values to
support this aspect as well, it would be good.