Allow "." in list item names of Args/Returns/Raises blocks

Blocks like this:

    Raises:
      tf.errors.OpError: Or one of its subclasses if an error occurs while
        creating the TensorFlow session.
      TypeError: If one of the arguments has the wrong type.

"tf.errors.OpError" is now parsed into a list item.

PiperOrigin-RevId: 170865165