bug#73117: 30.0.90; Imenu missing entries when flattening by group

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 73117@debbugs.gnu.org, Daniel Mendler <mail@daniel-mendler.de>
> From: Juri Linkov <juri@linkov.net>
> Date: Fri, 20 Sep 2024 09:58:24 +0300
> 
> > The following example configures `imenu-flatten" to "group" and then
> > shows the menu, first with `completions-format` set to "horizontal"
> > and then "vertical".
> >
> > ```elisp
> > (progn
> >   (setopt completions-group t)
> >   (setopt imenu-eager-completion-buffer nil)
> >   (setopt minibuffer-visible-completions t)
> >   (setopt imenu-flatten 'group)
> >
> >   (require 'imenu)
> >
> >   (dolist (format '(horizontal vertical))
> >     (setopt completions-format format)
> 
> Any fixes for completion--insert-horizontal should be pushed
> only to master, not to the release branch.
> 
> But for the release branch we should push the documentation fixes
> that will mention all prerequisites for this feature to work
> correctly:

Feel free to install on the release branch, and thanks.