message: Minor tf.function documentation update.
diff: @@ -752,6 +752,9 @@ def function(func=None,
  l.append(i) # Caution! Doesn't work.
  ```
  + Note that unlike other TensorFlow operations, we don't convert python
  + numerical inputs to tensors.
  +
  _Referencing `tf.Variable`s_
  The Python function `func` may reference stateful objects (such as
  