Add warning about messageIDs

See https://github.com/libp2p/specs/pull/285 and https://github.com/libp2p/go-libp2p-pubsub/issues/361

pb/rpc.proto

@@ -32,10 +32,12 @@ message ControlMessage {
 
 message ControlIHave {
 	optional string topicID = 1;
+	// implementors from other languages should use bytes here - go protobuf emits invalid utf8 strings
 	repeated string messageIDs = 2;
 }
 
 message ControlIWant {
+	// implementors from other languages should use bytes here - go protobuf emits invalid utf8 strings
 	repeated string messageIDs = 1;
 }