Document rederive_regex_groups

[kaihendry]

On start Bugzilla's checksetup.pl calls rederive_regex_groups which processes the user_group_map:
https://github.com/bugzilla/bugzilla/blob/e88ec7fdc2ea995c669a26af39be0edc46065f8b/Bugzilla/Install/DB.pm#L2947

We comment out the call from https://github.com/unee-t/bugzilla-customisation/blob/master/start since calling it would get the startup stuck, since Unee-T heavilty users user_group_map to manage permutations of user/unit/etc permissions & relationships.

I'm not sure on the cons of omitting this check. Hence the issue. If someone accidentally keeps this check in, you'll notice that Bugzilla will fail to start and the DB freaking like so: