authn_dbd docs: add reference to password formats note

(thanks Tom Browder on users@ for pointing out this is useful in trunk
docs but missing from 2.2).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1154782 13f79535-47bb-0310-9956-ffa450edef68