uac: docs - more details about execution of event_route[uac:reply]

(cherry picked from commit 1cec5e8)

src/modules/uac/doc/uac_admin.xml

@@ -921,9 +921,11 @@ failure_route[REMOTE_AUTH] {
 				<function moreinfo="none">event_route[uac:reply]</function>
 			</title>
 		<para>
-			Event route executed for the final reply to the request set with
-			uac_req_send(). The associated $uac_req(evroute) has to be set to
-			1.
+			Event route executed for the final reply of the branch for the
+			request sent with uac_req_send(). The associated $uac_req(evroute)
+			has to be set to 1. If the request is challenged for authentication
+			with 401/407, then the event_route is executed twice, first for
+			401/407 and second for final reply of the transaction.
 		</para>
 		<example>
 		<title><function>event_route[uac:reply]</function> usage</title>