> The shortdoc buffer currently lacks support for the outline-minor-mode.
> By setting the two variables outline-regexp and outline-level, we can
> unlock this feature. Does it make sense to provide this by default?
>
> (defun shortdoc--outline-level () (if (eq (char-after) ?\() 2 1)))
> (add-hook 'shortdoc-mode-hook
>           (lambda ()
>             (setq-local outline-level #'shortdoc--outline-level
>                         outline-regexp "[A-Z(]")))

Unfortunately, outline-regexp is not a reliable way to find
outline headings.  For this reason currently outlines in apropos
are broken: outline-regexp in apropos-mode is set to "^[^ \n]+",
that matches too many false positives, all blue lines below
are incorrectly identified as outline headings: