Update P4Runtime Spec to explicitly specify read-write symmetry excep…

…tion for backup replicas.

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

docs/v1/P4Runtime-Spec.mdk

@@ -2297,7 +2297,9 @@ preserved, it will be explicitly stated in this document. The
 `MessageDifferencer` class [@ProtoMessageDifferencer] included in the Protobuf
 C++ API supports comparing messages while treating repeated fields as sets, so
 that different orderings of the same elements are considered equal. This method
-of comparing Protobuf messages may come at a cost in performance.
+of comparing Protobuf messages may come at a cost in performance. 
+The `backup_replicas` must be returned in order when read to preserve the
+hierarchy of highest-preferred backup replicas.
 
 ### Data plane volatile objects { #sec-data-plane-volatile-objects }