nathelper: docs - rephrased the remarks about set_contact_alias()

(cherry picked from commit c009f27)
(cherry picked from commit f1bd6bb)
(cherry picked from commit c230842)

src/modules/nathelper/doc/nathelper_admin.xml

@@ -847,8 +847,10 @@ if(is_rfc1918("$rd")) {
 		<para>
 		Adds an <quote>;alias=ip~port~transport</quote> parameter to the
 		contact URI containing the received ip, port, and transport protocol.
-		The new contact URI is immediately visible to other modules in the
-		way the <function>fix_nated_contact()</function> does it.
+		The update of contact URI is signaled to a few other modules in the
+		way the <function>fix_nated_contact()</function> does it by using the
+		internal flags. The new value is not visible to pseudo-variables and it
+		does not change the SIP message buffer.
 		</para>
 		<para>Meaning of parameters:</para>
 		<itemizedlist>