Some more documentation in the material lib (#6617)

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

@@ -18,10 +18,11 @@ import 'raised_button.dart';
 ///
 /// See also:
 ///
-///  * [RaisedButton]
-///  * [FlatButton]
-///  * [Dialog]
-///  * [ButtonTheme]
+///  * [RaisedButton], a kind of button.
+///  * [FlatButton], another kind of button.
+///  * [Card], at the bottom of which it is common to place a [ButtonBar].
+///  * [Dialog], which uses a [ButtonBar] for its actions.
+///  * [ButtonTheme], which configures the [ButtonBar].
 class ButtonBar extends StatelessWidget {
   /// Creates a button bar.
   ///

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

@@ -14,7 +14,9 @@ import 'material.dart';
 /// See also:
 ///
 ///  * [ListItem], to display icons and text in a card.
-///  * [Dialog] and [showDialog], to display a modal card.
+///  * [ButtonBar], to display buttons at the bottom of a card. Typically these
+///    would be styled using a ButtonTheme created with [new ButtonTheme.bar].
+///  * [showDialog], to display a modal card.
 ///  * <https://material.google.com/components/cards.html>
 class Card extends StatelessWidget {
   /// Creates a material design card.