document PROTO_FLAG_IMMEDIATE

DESIGN

@@ -132,5 +132,9 @@ set, the protocol handler needs to clean up immediately as good as possible,
 without waiting for its pending actions to complete. If it has spawned
 any child processes, it needs to kill them and clean up their mess.
 
+Simple protocol handlers can set the PROTO_FLAG_IMMEDIATE flag if they
+can perform all required actions immediately without waiting and thus
+do not need to schedule IFPEV_UP and IFPEV_DOWN transitions. This will
+cause those events to be generated by core code instead.
 
 ## TODO: Configuration management, ubus callbacks