unconfined: add a note about DBUS

Addresses https://github.com/SELinuxProject/refpolicy/issues/18

policy/modules/system/unconfined.if

@@ -118,6 +118,11 @@ interface(`unconfined_domain_noaudit',`
 ##	<p>
 ##	Only completely trusted domains should use this interface.
 ##	</p>
+##	<p>
+##	Does not allow return communications from confined
+##	domains via message based mechanisms such as dbus or
+##	SysV message queues.
+##	</p>
 ## </desc>
 ## <param name="domain">
 ##	<summary>