Update the P4RuntimeSpec to specify examples of packet processing sid…

…e effects

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

docs/v1/P4Runtime-Spec.mdk

@@ -4578,7 +4578,9 @@ 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.
+after-multicast-replication processing. Note that the packet processing side
+effects include counter updates, meter updates, and any P4 register array
+writes in the P4 code after the multicast replication is done.
 
 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