Add documentation to 2 DeprecatedPaintLayer objects

Source/core/paint/DeprecatedPaintLayerStackingNode.h

Index: Source/core/paint/DeprecatedPaintLayerStackingNode.h
diff --git a/Source/core/paint/DeprecatedPaintLayerStackingNode.h b/Source/core/paint/DeprecatedPaintLayerStackingNode.h
index d4b0150ca1e833351e9e4222832cacb55e4e7a24..cad1d7d44939e0e40f3cbca47247264d1a0c971a 100644
--- a/Source/core/paint/DeprecatedPaintLayerStackingNode.h
+++ b/Source/core/paint/DeprecatedPaintLayerStackingNode.h
@@ -58,6 +58,33 @@ class DeprecatedPaintLayerCompositor;
 class ComputedStyle;
 class LayoutBoxModelObject;
 
+// DeprecatedPaintLayerStackingNode represents anything that is treated as a
+// stacking context.
+//
+// Stacking contexts are the basis for the CSS painting algorithm. The paint
+// order is determined by walking stacking contexts (or elements treated like a
+// stacking context like positioned objects or floats) in an order defined by
+// ‘z-index’. This walk is interleaved with content that is not a stacking.
+// context. See CSS 2.1 appendix E for the actual algorithm
+// http://www.w3.org/TR/CSS21/zindex.html
+// See also DeprecatedPaintLayerPainter (in particular paintLayerContents) for
+// our implementation of the walk.
+//
+// Stacking contexts form a subtree over the layout tree. Ideally we would want
+// objects of this class to be a node in this tree but there are potential
+// issues with stale pointers so we rely on DeprecatedPaintLayer's tree
+// structure.
+//
+// This class's purpose is to represent a node in the stacking context tree
+// (aka paint tree). It currently caches the z-order lists for painting and
+// hit-testing.
+//
+// To implement any z-order list iterations, use
+// DeprecatedPaintLayerStackingNodeIterator and
+// DeprecatedPaintLayerStackingNodeReverseIterator.
+//
+// This class is NOT DEPRECATED, DeprecatedPaintLayer is and we match its
+// naming.
 class CORE_EXPORT DeprecatedPaintLayerStackingNode {
     WTF_MAKE_FAST_ALLOCATED(DeprecatedPaintLayerStackingNode);
     WTF_MAKE_NONCOPYABLE(DeprecatedPaintLayerStackingNode);