MAN: Add note about AD Group types

Linux admins/users may not know that the AD distribution group type
is intended only for email. Per microsoft: Distribution groups are
not security enabled, which means that they cannot be listed in
discretionary access control lists (DACLs).

src/man/sssd-ad.5.xml

@@ -109,6 +109,10 @@ ldap_id_mapping = False
             case-insensitive in the AD provider for compatibility with Active
             Directory's LDAP implementation.
         </para>
+        <para>
+            SSSD only resolves Security-enabled Active Directory group types.
+            (e.g. Not <quote>distribution</quote> groups)
+        </para>
     </refsect1>
 
     <refsect1 id='configuration-options'>