message: Mention py_function in tf.data, fix some typos.
diff: @@ -1057,6 +1057,17 @@ class DatasetV2(tracking_base.Trackable):
  In addition to `tf.Tensor` objects, `map_func` can accept as arguments and
  return `tf.SparseTensor` objects.
  + Note that irrespective of the context in which `map_func` is defined (eager
  + vs. graph), tf.data traces the function and executes it as a graph. To use
  + Python code inside of the function you have two options:
  +
  + 1) Rely on AutoGraph to convert Python code into an equivalent graph
  + computation. The downside of this approach is that AutoGraph can convert
  + some but not all Python code.
  +
  + 2) Use `tf.py_function`, which allows you to write arbitrary Python code but
  + will generally result in worse performance than 1).
  +
  Args:
  map_func: A function mapping a nested structure of tensors (having
  shapes and types defined by `self.output_shapes` and
  