Add documentation about configuration default value.

Author: mono0926

packages/animations/lib/src/modal.dart

@@ -29,6 +29,7 @@ typedef _ModalTransitionBuilder = Widget Function(
 /// modal route that will be displayed, such as the enter and exit
 /// transitions, the duration of the transitions, and modal barrier
 /// properties.
+/// By default, `configuration` is [FadeScaleTransitionConfiguration].
 ///
 /// The `useRootNavigator` argument is used to determine whether to push the
 /// modal to the [Navigator] furthest from or nearest to the given `context`.