message: Add detail to set_learning_phase documentation
diff: @@ -392,6 +392,20 @@ def _default_learning_phase():
  def set_learning_phase(value):
  """Sets the learning phase to a fixed value.
  + The backend learning phase affects any code that calls
  + `backend.learning_phase()`
  + In particular, all Keras built-in layers use the learning phase as the default
  + for the `training` arg to `Layer.__call__`.
  +
  + User-written layers and models can achieve the same behavior with code that
  + looks like:
  +
  + ```python
  + def call(self, inputs, training=None):
  + if training is None:
  + training = backend.learning_phase()
  + ```
  +
  Arguments:
  value: Learning phase value, either 0 or 1 (integers).
  0 = test, 1 = train
  