bug#50394: Improve "Completion List Mode" NEWS entry

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Sun, 5 Sep 2021 07:27:50 +0200
> Cc: gregory heytings <ghe@sdf.org>
> 
> ** Completion List Mode
> New key bindings have been added: 'n' and 'p' to navigate completions,
> and 'M-g M-c' to switch to the minibuffer, and you can also switch back
> to the completion list buffer with 'M-g M-c'.
> 
> I suspect that most users have no idea what is "completion list mode",

If they don't, they could just do

  C-h f completion-list-mode RET

and read there.

> as IIUC it is an implementation detail.

??? No, it's a complete major mode, according to its doc string.