MAN: Document that POSIX attributes must be replicated to GC

Currently the AD provider relies on the presence of the POSIX attributes
in the Global Catalog. This patch mentiones the fact in the sssd-ad(5)
manual page.

src/man/sssd-ad.5.xml

@@ -73,6 +73,11 @@
             <programlisting>
 ldap_id_mapping = False
             </programlisting>
+            In order to retrieve users and groups using POSIX attributes from trusted
+            domains, the AD administrator must make sure that the POSIX attributes
+            are replicated to the Global Catalog.
+        </para>
+        <para>
             Users, groups and other entities served by SSSD are always treated as
             case-insensitive in the AD provider for compatibility with Active
             Directory's LDAP implementation.