* doc/emacs/maintaining.texi (Bug Reference): Add section explaining
adding support for third-party packages.
* lisp/progmodes/bug-reference.el (bug-reference-maybe-setup-from-vc)
(bug-reference-maybe-setup-from-mail)
(bug-reference-maybe-setup-from-irc): Rename from
bug-reference--maybe-setup-from-*, i.e., un-privatize them because
they are advertised in the docs now.