message: Improve docs for TypeSpec
diff: @@ -57,9 +57,18 @@ class TypeSpec(object):
  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
  + 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.
  +
  + Example:
  +
  + >>> spec = tf.RaggedTensorSpec(shape=[None, None], dtype=tf.int32)
  + >>> @tf.function(input_signature=[spec])
  + ... def double(x):
  + ... return x * 2
  + >>> print(double(tf.ragged.constant([[1, 2], [3]])))
  + <tf.RaggedTensor [[2, 4], [6]]>
  """
  # === Subclassing ===
  #
  