message: Add note that creating new subclasses of TypeSpec (outside of TensorFlow core) is not currently supported.
diff: @@ -49,6 +49,13 @@ class TypeSpec(object):
  A `tf.TypeSpec` provides metadata describing an object accepted or returned
  by TensorFlow APIs. Concrete subclasses, such as `tf.TensorSpec` and
  `tf.RaggedTensorSpec`, are used to describe different value types.
  +
  + For example, `tf.function`'s `input_signature` argument accepts a list
  + (or nested structure) of `TypeSpec`s.
  +
  + Creating new subclasses of TypeSpec (outside of TensorFlow core) is not
  + currently supported. In particular, we may make breaking changes to the
  + private methods and properties defined by this base class.
  """
  # === Subclassing ===
  #
  