Docs clarification for how RTL/LTR locale affects ButtonBar (#24264)

packages/flutter/lib/src/material/button_bar.dart

@@ -9,10 +9,14 @@ import 'dialog.dart';
 import 'flat_button.dart';
 import 'raised_button.dart';
 
-/// A horizontal arrangement of buttons.
+/// An end-aligned row of buttons.
 ///
 /// Places the buttons horizontally according to the padding in the current
-/// [ButtonTheme].
+/// [ButtonTheme]. The children are laid out in a [Row] with
+/// [MainAxisAlignment.end]. When the [Directionality] is [TextDirection.ltr],
+/// the button bar's children are right justified and the last child becomes
+/// the rightmost child. When the [Directionality] [TextDirection.rtl] the
+/// children are left justified and the last child becomes the leftmost child.
 ///
 /// Used by [Dialog] to arrange the actions at the bottom of the dialog.
 ///