rtpengine: added note about rtpproxy_manage() afte t_continue()

- or after any other function that does suspend/resume of the
  transaction
- it behaves as used in failure_route
- related to GH #855

modules/rtpengine/doc/rtpengine_admin.xml

@@ -1091,7 +1091,12 @@ rtpengine_delete();
 		</listitem>
 		<listitem>
 			<para>
-			If BYE or CANCEL, or called within a FAILURE_ROUTE[], then do <function>rtpengine_delete()</function>
+			If BYE or CANCEL, or called within a FAILURE_ROUTE[], then call
+			<function>rtpengine_delete()</function>. Be careful with calling
+			this function after resuming a suspended transaction (e.g., after
+			t_continue()), because the context of executed route is FAILURE
+			ROUTE (in other words, rtpengine_manage() in the route block of
+			t_continue() does the same as in failure_route).
 			</para>
 		</listitem>
 		<listitem>