message: Add definition of what PS stands for
diff: @@ -2708,6 +2708,19 @@ def create_local_cluster(num_workers,
  ps_config=None):
  """Create and start local servers and return the associated `Server` objects.
  + "PS" stands for "parameter server": a task responsible for storing and
  + updating the model's parameters. Other tasks send updates to these parameters
  + as they work on optimizing the parameters. This particular division of labor
  + between tasks is not required, but is common for distributed training.
  +
  + Read more at https://www.tensorflow.org/guide/extend/architecture
  +
  + TODO: image from https://www.tensorflow.org/images/diag1.svg
  +
  + Figure 2 illustrates the interaction of these components.
  + "/job:worker/task:0" and "/job:ps/task:0" are both tasks with worker services.
  +
  +
  Example:
  ```python
  workers, _ = tf.test.create_local_cluster(num_workers=2, num_ps=2)
  