Improve tf.train.list_variables API doc.

PiperOrigin-RevId: 320285328
Change-Id: I52d1d61e2c181259699024a8197d5364d3ba747e

tensorflow/python/training/checkpoint_utils.py

@@ -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()