bug#64227: Setting of outline levels for custom headings

[uzibalqa via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

I am using outline-minor-mode and have changed outline-regexp, outline-heading-alist.

Have seen that for some major modes (e.g. emacs-lisp-mode), with outline-minor-mode-highlight
set to 'override  nil), one has to call (setq-local outline-level 'outline-level) for the 
heading faces to take effect.

But without the call to outline-minor-mode-highlight, the headings defined by outline-regexp
still get recognised, but not the outline levels.

It is not evident at this point, which major modes require a call to (setq-local outline-level 'outline-level).
There is not enough information in the documentation to get a custom headings setup operational.

Furthermore, although a call to (setq-local outline-level 'outline-level) solved the problem,
a better approach would be for outline-minor-mode to automatically detect whether a custom heading
was specified, and thus act accordingly.  It would be quite a struggle for people to perceive the
need to call (setq-local outline-level 'outline-level), as the change is only needed for certain
major modes only.