message: Improve tf.train.list_variables API doc.
diff: @@ -87,13 +87,27 @@ def load_variable(ckpt_dir_or_file, name):
  @tf_export("train.list_variables")
  def list_variables(ckpt_dir_or_file):
  - """Returns list of all variables in the checkpoint.
  + """Lists the checkpoint keys and shapes of variables in a checkpoint.
  +
  + Checkpoint keys are paths in a checkpoint graph.
  +
  + Example usage:
  +
  + ```python
  + import tensorflow as tf
  + import os
  + ckpt_directory = "/tmp/training_checkpoints/ckpt"
  + ckpt = tf.train.Checkpoint(optimizer=optimizer, model=model)
  + manager = tf.train.CheckpointManager(ckpt, ckpt_directory, max_to_keep=3)
  + train_and_checkpoint(model, manager)
  + tf.train.list_variables(manager.latest_checkpoint)
  + ```
  Args:
  ckpt_dir_or_file: Directory with checkpoints file or path to checkpoint.
  Returns:
  - List of tuples `(name, shape)`.
  + List of tuples `(key, shape)`.
  """
  reader = load_checkpoint(ckpt_dir_or_file)
  variable_map = reader.get_variable_to_shape_map()
  