Added a feature to allow you to use a regex to match an LDAP member
field. But upon further review, I don't think it would ever be
needed unless someone's LDAP was really mis-configured. So this
commit is for posterity only.
(cherry picked from commit f5b2b5c7d0a02577f36782bebfdc5ff2082f9951)

Adding back docs for this feature so they fit with the docs
branch this was pulled from.