* lisp/outline.el (outline-search-function): New variable (bug#53981).

* lisp/outline.el (outline-search-function): New variable (bug#53981).

(outline-font-lock-keywords, outline-font-lock-face)
(outline-minor-mode-highlight-buffer, outline-next-preface)
(outline-next-heading, outline-previous-heading)
(outline-back-to-heading, outline-on-heading-p, outline-demote)
(outline-map-region, outline-next-visible-heading)
(outline-hide-sublevels, outline-up-heading): Use outline-search-function
when it's non-nil as an alternative to searching outline-regexp.
(outline-search-level, outline-search-text-property): New functions.

* lisp/apropos.el (apropos-mode): Set outline-search-function
instead of unreliable outline-regexp.
(apropos-print): Add text property outline-level.

* lisp/emacs-lisp/shortdoc.el (shortdoc-display-group):
Add text property outline-level on text separate from final newlines.
(shortdoc-display-group): Add a narrow newline to not show
text properties of the final line when the outline is hidden.
(shortdoc--display-function): Add text property outline-level.
(shortdoc-mode): Set buffer-local outline-search-function and outline-level.