Adds API documentation for SkyGames

sky/examples/game/lib/sprite_widget.dart

 part of sprites;
 
+/// A widget that uses a [SpriteBox] to render a sprite node tree to the screen.
 class SpriteWidget extends OneChildRenderObjectWrapper {
 
+  /// The rootNode of the sprite node tree.
+  ///
+  ///     var node = mySpriteWidget.rootNode;
   final NodeWithSize rootNode;
+
+  /// The transform mode used to fit the sprite node tree to the size of the widget.
   final SpriteBoxTransformMode transformMode;
 
+  /// Creates a new sprite widget with [rootNode] as its content.
+  ///
+  /// The widget will setup the coordinate space for the sprite node tree using the size of the [rootNode] in
+  /// combination with the supplied [transformMode]. By default the letterbox transform mode is used. See
+  /// [SpriteBoxTransformMode] for more details on the different modes.
+  ///
+  /// The most common way to setup the sprite node graph is to subclass [NodeWithSize] and pass it to the sprite widget.
+  /// In the custom subclass it's possible to build the node graph, do animations and handle user events.
+  ///
+  ///     var mySpriteTree = new MyCustomNodeWithSize();
+  ///     var mySpriteWidget = new SpriteWidget(mySpriteTree, SpriteBoxTransformMode.fixedHeight);
   SpriteWidget(this.rootNode, [this.transformMode = SpriteBoxTransformMode.letterbox]);
 
   SpriteBox get root => super.root;
 
   SpriteBox createNode() => new SpriteBox(rootNode, transformMode);
 
   void syncRenderObject(SpriteWidget old) {
     super.syncRenderObject(old);
 
     // SpriteBox doesn't allow mutation of these properties
     assert(rootNode == root.rootNode);
-    assert(transformMode == root.transformMode);
+    assert(transformMode == root._transformMode);
   }
 }