message: Add a link to the guide.
  Fixes
diff: @@ -766,6 +766,8 @@ class Model(base_layer.Layer, version_utils.ModelVersionSelector):
  """The logic for one training step.
  This method can be overridden to support custom training logic.
  + For concrete examples of how to override this method see
  + [Customizing what happends in fit](https://www.tensorflow.org/guide/keras/customizing_what_happens_in_fit).
  This method is called by `Model.make_train_function`.
  This method should contain the mathematical logic for one step of training.
  