registrar: note about empty value for received_param

(cherry picked from commit e35fe9c6095361414565b1099dea1ad5950fb38d)

modules/registrar/README

@@ -441,7 +441,8 @@ modparam("registrar", "received_avp", "$avp(s:rcv)")
 3.11. received_param (string)
 
    The name of the parameter that will be appended to Contacts of 200 OK
-   when the received URI was set by nathelper module.
+   when the received URI was set by nathelper module. If the value is
+   empty string, then the parameter is not appended anymore.
 
    Default value is "received".
 

modules/registrar/doc/registrar_admin.xml

@@ -364,7 +364,8 @@ modparam("registrar", "received_avp", "$avp(s:rcv)")
 		<title><varname>received_param</varname> (string)</title>
 		<para>
 		The name of the parameter that will be appended to Contacts of 
-		200 OK when the received URI was set by nathelper module.
+		200 OK when the received URI was set by nathelper module. If the
+		value is empty string, then the parameter is not appended anymore.
 		</para>
 		<para>
 		<emphasis>