>>> +                          (re-search-forward
>>> +			   (concat "^\\(?:" outline-regexp "\\)")
>>> +			   nil 'move)))
>>
>> These two loops cons a new string each iteration.  (So did the
>> original code, but if we are touching this, might as well fix that.)
>
> This is optimized as well:

Here is a more tested patch that works in apropos and shortdoc.

Also tested for group outlines in the Completions buffer with:

```
(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))
```

It even works when using the search function that searches for
outline-regexp.  This better shows the meaning of its arguments:

```
(setq-default
 outline-search-function
 (lambda (&optional bound move backward looking-at)
   (cond
    (looking-at (looking-at outline-regexp))
    (backward
     (re-search-backward
      (concat "^\\(?:" outline-regexp "\\).*")
      bound (if move 'move t)))
    (t
     (re-search-forward
      (concat "^\\(?:" outline-regexp "\\).*")
      bound (if move 'move t)))))
 outline-level
 (lambda () (looking-at outline-regexp) (outline-level)))
```

As can be seen, the default outline-level function can't be used,
because the search function is expected to match to the end
of the heading line, but the default outline-level expects
to match only beginning of the outline heading.