bug#52389: 29.0.50; Imenu does not pop up the Completions eagerly

[Eli Zaretskii]

> From: Protesilaos Stavrou <info@protesilaos.com>
> Date: Fri, 17 Dec 2021 13:57:52 +0200
> Cc: Lars Ingebrigtsen <larsi@gnus.org>, 52389@debbugs.gnu.org,
>  Daniel Martín <mardani29@yahoo.es>
> 
> +(defcustom imenu-no-eager-completion-buffer t
> +  "If non-nil, do not eagerly popup the completion buffer."

The doc string should go on to explain what does it mean to "eagerly
popup a buffer".  It is not self-evident.

(Yes, I know this was in the original text, but while we are at that,
we might as well fix that omission.)

Thanks.