Fill out some more documentation about building RenderBox subclasses.

sky/sdk/lib/rendering/box.dart

Index: sky/sdk/lib/rendering/box.dart
diff --git a/sky/sdk/lib/rendering/box.dart b/sky/sdk/lib/rendering/box.dart
index 0d0b624c4e88f8fb9830994ff4d19c44638b0e7f..b5a961340538f3dcc39215d828648c7ab653c0ba 100644
--- a/sky/sdk/lib/rendering/box.dart
+++ b/sky/sdk/lib/rendering/box.dart
@@ -1133,9 +1133,12 @@ class RenderView extends RenderObject with RenderObjectWithChildMixin<RenderBox>
 
 }
 
-// DEFAULT BEHAVIORS FOR RENDERBOX CONTAINERS
+// HELPER METHODS FOR RENDERBOX CONTAINERS
 abstract class RenderBoxContainerDefaultsMixin<ChildType extends RenderBox, ParentDataType extends ContainerParentDataMixin<ChildType>> implements ContainerRenderObjectMixin<ChildType, ParentDataType> {
 
+  // This class, by convention, doesn't override any members of the superclass.
+  // It only provides helper functions that subclasses can call.
+
   double defaultGetDistanceToFirstActualBaseline(TextBaseline baseline) {
     assert(!needsLayout);
     RenderBox child = firstChild;