message: Add missing exception to Model.fit() docstring
  The shuffle argument to the fit() method in the Model class gets ignored
  if the input x to the method is a tf.data.Dataset object. But this isn't
  documented in the docstring of the method.
diff: @@ -949,7 +949,8 @@ class Model(base_layer.Layer, version_utils.ModelVersionSelector):
  are supported in `x`, eg, dict, generator or `keras.utils.Sequence`.
  shuffle: Boolean (whether to shuffle the training data
  before each epoch) or str (for 'batch'). This argument is ignored
  - when `x` is a generator. 'batch' is a special option for dealing
  + when `x` is a generator or an object of tf.data.Dataset.
  + 'batch' is a special option for dealing
  with the limitations of HDF5 data; it shuffles in batch-sized
  chunks. Has no effect when `steps_per_epoch` is not `None`.
  class_weight: Optional dictionary mapping class indices (integers)
  