auth_samba4: Describe the slightly unusual role of auth_samba4

Reviewed-by: Jelmer Vernooij <jelmer@samba.org>
Signed-off-by: Andrew Bartlett <abartlet@samba.org>

Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Mon Dec 16 03:18:00 CET 2013 on sn-devel-104