message: Document limitations of SecretProto
diff: @@ -45,6 +45,9 @@ inline google::protobuf::ArenaOptions SecretArenaOptions() {
  // Stores secret (sensitive) protobuf and makes sure it's marked as such and
  // destroyed in a safe way.
  +//
  +// Note: Currently does not protect fields of type "string" and "bytes"
  +// (depends on https://github.com/protocolbuffers/protobuf/issues/1896)
  template <typename T>
  class SecretProto {
  public:
  