Update the P4RuntimeSpec to specify packet processing side effects of…

… a fully down replica + it's backups

Signed-off-by: Matthew Lam <[email protected]>

docs/v1/P4Runtime-Spec.mdk

@@ -4571,7 +4571,14 @@ uses the first backup replica for which its port is up. Whenever the primary
 port goes back up, the system will use the primary port again. In the case
 when the primary replica's ports and the backup replicas' ports are down,
 nothing is done for recovery until the primary replica or the backup replicas's
-ports go back up.
+ports go back up. When the primary and all the backups are down for a
+particular replica, the packet processing side effects can result in one of the
+two main expected behaviors that the user is recommended to implement.
+In the first case, no replica is created so none of the side effects from the
+after-multicast-replication processing should occur for this replica. In the
+second case, the system sends one replica to the primary port, which gets
+dropped, but the system will perform any side effects of the
+after-multicast-replication processing.
 
 The egress packets may be distinguished for further processing in the egress
 using the `instance` metadata. Note that a packet may not be both unicast and