message: protobuf: add comment to MessageUtil to include .pb.validate.h
diff: @@ -118,6 +118,8 @@ public:
  /**
  * Validate protoc-gen-validate constraints on a given protobuf.
  + * Note the corresponding `.pb.validate.h` for the message has to be included in the source file
  + * of caller.
  * @param message message to validate.
  * @throw ProtoValidationException if the message does not satisfy its type constraints.
  */
  @@ -136,6 +138,8 @@ public:
  /**
  * Downcast and validate protoc-gen-validate constraints on a given protobuf.
  + * Note the corresponding `.pb.validate.h` for the message has to be included in the source file
  + * of caller.
  * @param message const Protobuf::Message& to downcast and validate.
  * @return const MessageType& the concrete message type downcasted to on success.
  * @throw ProtoValidationException if the message does not satisfy its type constraints.
  