message: Clarify that these error codes are the same that are used in Protobuf and gRPC.
diff: @@ -17,6 +17,8 @@ option go_package = "github.com/tensorflow/tensorflow/tensorflow/go/core/protobu
  // - Changes to this list should only be made if there is a compelling
  // need that can't be satisfied in another way. Such changes
  // must be approved by at least two OWNERS.
  +// - These error codes must match gRPC and protobuf error codes (except for
  +// DO_NOT_USE_RESERVED_FOR_FUTURE_EXPANSION_USE_DEFAULT_IN_SWITCH_INSTEAD_).
  //
  // Sometimes multiple error codes may apply. Services should return
  // the most specific error code that applies. For example, prefer
  