message: Add comments to SecretValue.
diff: @@ -112,6 +112,12 @@ inline SecretData SecretDataFromStringView(absl::string_view secret) {
  return {secret.begin(), secret.end()};
  }
  +// The same as SecretUniquePtr, but with value semantics.
  +//
  +// NOTE: SecretValue<T> will only protect the data which is stored in the
  +// memory which a T object takes on the stack. In particular, std::string and
  +// std::vector SHOULD NOT be used as arguments of T: they allocate memory
  +// on the heap, and hence the data stored in them will NOT be protected.
  template <typename T>
  class SecretValue {
  public:
  