AuthbasicProvider of something other then LDAP, with AuthLDAPURL in the per-dir config

behaves a little unintuitively. Document the behavior if/until LDAP can better step
out of the way during authorization.

PR#45946



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@701648 13f79535-47bb-0310-9956-ffa450edef68