message: Clarify that MirroredStrategy(devices=None) detects GPUs.
diff: @@ -429,7 +429,8 @@ class MirroredStrategy(distribute_lib.Strategy):
  The multi-worker version will be added in the future.
  Args:
  - devices: a list of device strings.
  + devices: a list of device strings. If `None`, all available GPUs are used.
  + If no GPUs are found, CPU is used.
  cross_device_ops: optional, a descedant of `CrossDeviceOps`. If this is not
  set, nccl will be use by default.
  """
  