Update pydoc for the issue of using Keras BatchNorm in TFF.

PiperOrigin-RevId: 402693657

tensorflow_federated/python/learning/keras_utils.py

@@ -58,6 +58,11 @@ def from_keras_model(
  guaranteed to exist through the functional or Sequential API but are
  not necessarily present for subclassed models.
 
+ Note: This function raises a UserWarning if the `tf.keras.Model` contains a
+ BatchNormalization layer, as the batch mean and variance will be treated as
+ non-trainable variables and won't be updated during the training (see
+ b/186845846 for more information). Consider using Group Normalization instead.
+
  Args:
  keras_model: A `tf.keras.Model` object that is not compiled.
  loss: A single `tf.keras.losses.Loss` or a list of losses-per-output. If a