drouting docs: Clarify behavior for the gateway "type" parameter

(cherry picked from commit 9cf184e)

modules/drouting/doc/drouting_admin.xml

@@ -148,8 +148,8 @@
 	<itemizedlist>
 	<listitem><para>id (string)</para></listitem>
 	<listitem><para>SIP address (SIP URI)</para></listitem>
-	<listitem><para>type (number to allow to group GW based on purpose,
-	like inbound, outbound, etc)</para></listitem>
+	<listitem><para>type (positive integer which allows GWs to be grouped by purpose,
+	e.g. inbound, outbound, etc.)</para></listitem>
 	<listitem>strip value (number of digits) from dialled
 	number<para></para></listitem>
 	<listitem><para>prefix (string) to be added to dialled
@@ -1239,7 +1239,7 @@ if (use_next_gw(, , "$var(carrier_attrs)")) {
 					<para>
 					<emphasis role='bold'>type</emphasis> (optional) - GW/destination 
 					type to be checked; when ommiting this parameter or specifying
-					a negative value i.e. "-1", matching will be done against all groups
+					a special value ("-1" or "0"), matching will be done against all groups
 					</para>
 				</listitem>
 				<listitem>
@@ -1322,7 +1322,7 @@ if (goes_to_gw("1", , "$var(gw_attrs)")) {
 					<para>
 					<emphasis role='bold'>type</emphasis> (optional) - GW/destination
 					type to be checked; when ommiting this parameter or specifying
-					a negative value i.e. "-1", matching will be done against all groups
+					a special value ("-1" or "0"), matching will be done against all groups
 					</para>
 				</listitem>
 				<listitem>
@@ -1408,7 +1408,7 @@ if (is_from_gw("3","1") {
 					<para>
 					<emphasis role='bold'>type</emphasis> (optional) - GW/destination
 					type to be checked; when ommiting this parameter or specifying
-					a negative value i.e. "-1", matching will be done against all groups
+					a special value ("-1" or "0"), matching will be done against all groups
 					</para>
 				</listitem>
 				<listitem>