message: [tf.data] Improve TFRecordWriter doc.
  Fixes
diff: @@ -67,6 +67,10 @@ class TFRecordWriter(object):
  dataset = dataset.apply(tf.data.experimental.group_by_window(
  lambda i, _: i % NUM_SHARDS, reduce_func, tf.int64.max
  ))
  +
  + # Iterate through the dataset to trigger data writing.
  + for _ in dataset:
  + pass
  ```
  """
  