Eli Zaretskii <eliz@gnu.org> writes:

> Thanks, but please add a NEWS entry announcing this, and also mention
> this variant in the user manual, where completion-preview-mode is
> documented.

All right, I'm attaching an updated patch below.  Since
'completion-preview-mode' is new in Emacs 30, I extended the existing
NEWS entry with a mention of the global variant, instead of adding
another entry.  I've also renamed the global variant per Juri's comment: