uac: extend docs for uac_auth() example, only t_relay() on success

- extend docs for uac_auth() example, only t_relay() on success
- otherwise you will get a 'no branches for forwarding' error from tm

(cherry picked from commit 668402e)

src/modules/uac/doc/uac_admin.xml

@@ -836,8 +836,9 @@ failure_route[TRUNKAUTH] {
     if(t_check_status("401|407")) {
         $avp(auser) = "test";
         $avp(apass) = "test";
-        uac_auth();
-        t_relay();
+        if (uac_auth()) {
+            t_relay();
+        }
         exit;
     }
 }