[TASK][DOC] Additional info for Bind DN using Active Directory

Change-Id: I8230623a5842249ebe992a7945921e23ed28c3ef
Reviewed-on: http://review.typo3.org/32242
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>