RTPProxy: Documentation improvements

- added a note about compatibility with different implementations for the "x"-flag (namely RFC 4091 and RFC 6157)
- made more clear, that "x" is only a shortcut for the "IE" and "EI" flags of RTPProxy

modules/rtpproxy/doc/rtpproxy_admin.xml

@@ -343,11 +343,17 @@ rtpproxy_offer();
 				the 'w' flag for clients behind NAT! See also above notes!
 				</para></listitem>
 				<listitem><para>
-				<emphasis>x</emphasis> - this flag will do automatic bridging between IPv4 on the
+				<emphasis>x</emphasis> - this flag a shortcut for using the "ie" or "ei"-flags of RTP-Proxy,
+				in order to do automatic bridging between IPv4 on the
 				"internal network" and IPv6 on the "external network". The distinction is done by
 				the given IP in the SDP, e.g. a IPv4 Address will always call "ie" to the RTPProxy
 				(IPv4(i) to IPv6(e)) and an IPv6Address will always call "ei" to the RTPProxy (IPv6(e)
 				to IPv4(i)).
+				</para><para>
+				Note: Please note, that this will only work properly with non-dual-stack user-agents or with
+				dual-stack clients according to RFC6157 (which suggest ICE for Dual-Stack implementations).
+				This short-cut will not work properly with RFC4091 (ANAT) compatible clients, which suggests
+				having different m-lines with different IP-protocols grouped together.
 				</para></listitem>
 				<listitem><para>
 				<emphasis>f</emphasis> - instructs rtpproxy to ignore marks