bug#53981: 28.0.91; shortdoc: Add support for outline-minor-mode

[Juri Linkov]

close 53981 29.0.50
thanks

> Here is a more tested patch that works in apropos and shortdoc.
>
> Also tested for group outlines in the Completions buffer with:
>
> ```
> (add-hook 'completion-list-mode-hook
>           (lambda ()
>             (setq-local
>              outline-search-function
>              (lambda (&optional bound move backward looking-at)
>                (outline-search-text-property
>                 'face 'completions-group-separator
>                 bound move backward looking-at))
>              outline-level (lambda () 1))
> ```

Now pushed to master and closed.