add doc to explain call structure and how to override

python/mxnet/gluon/contrib/estimator/estimator.py

@@ -228,7 +228,11 @@ def evaluate(self,
                  val_data,
                  val_metrics,
                  batch_axis=0):
-        """Evaluate model on validation data
+        """Evaluate model on validation data.
+
+        This function calls :py:func:`evaluate_batch` on each of the batches from the
+        validation data loader. Thus, for custom use cases, it's possible to inherit the
+        estimator class and override :py:func:`evaluate_batch`.
 
         Parameters
         ----------
@@ -284,6 +288,10 @@ def fit(self, train_data,
         number of epochs or batches. The batch size is inferred from the
         data loader's batch_size.
 
+        This function calls :py:func:`fit_batch` on each of the batches from the
+        training data loader. Thus, for custom use cases, it's possible to inherit the
+        estimator class and override :py:func:`fit_batch`.
+
         Parameters
         ----------
         train_data : DataLoader