[TASK][DOC] How to mirror a hierarchy of LDAP groups

This mechanism does not seem to be standard but the corresponding lines
of code are now documented.

Resolves: #60971
Change-Id: I60b9d01f3691274eee085b0e466b34ff57abb383
Reviewed-on: http://review.typo3.org/32195
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>