[+] add more guidance on when to use PATH_ACK frames

draft-ietf-quic-multipath.md

@@ -319,7 +319,11 @@ Path ID 0.
 After the handshake concluded if negotiation of multipath support succeeded,
 endpoints SHOULD use PATH_ACK frames instead of ACK frames,
 also for acknowledging so far unacknowledged 0-RTT packets, using
-Path ID 0. Similarly after a successful handshake, endpoints SHOULD also use
+Path ID 0. Further more, endpoints SHOULD use PATH_ACK frames in all 1-RTT packets after
+the successful negotiation for multipath. If endpoints receive ACK frame 
+after a succesful negotiation resulting from packet out-of-order arrival, 
+endpoints could consider it as used for initial PATH ID 0 without causing connection error. 
+Similarly after a successful handshake, endpoints SHOULD also use
 the PATH_NEW_CONNECTION_ID frame to provide new connection IDs for Path ID 0 and,
 respectively, the PATH_RETIRE_CONNECTION_ID frame to retire connection IDs for
 Path ID 0.