Add sample code for describeSemanticsConfiguration (#12830)

packages/flutter/lib/src/rendering/object.dart

@@ -2125,6 +2125,28 @@ abstract class RenderObject extends AbstractNode with DiagnosticableTreeMixin im
   /// [describeSemanticsConfiguration] call (for example as a result of
   /// asynchronous computation) will at best have no useful effect and at worse
   /// will cause crashes as the data will be in an inconsistent state.
+  ///
+  /// ## Sample code
+  ///
+  /// The following snippet will describe the node as a button that responds to
+  /// tap actions.
+  ///
+  /// ```dart
+  /// abstract class SemanticButtonRenderObject extends RenderObject {
+  ///   @override
+  ///   void describeSemanticsConfiguration(SemanticsConfiguration config) {
+  ///     super.describeSemanticsConfiguration(config);
+  ///     config
+  ///       ..addAction(SemanticsAction.tap, _handleTap)
+  ///       ..label = 'I am a button'
+  ///       ..isButton = true;
+  ///   }
+  ///
+  ///   void _handleTap() {
+  ///     // Do something.
+  ///   }
+  /// }
+  /// ```
   @protected
   void describeSemanticsConfiguration(SemanticsConfiguration config) {
     // Nothing to do by default.