Add documentation to 2 DeprecatedPaintLayer objects

Source/core/paint/DeprecatedPaintLayer.h

Index: Source/core/paint/DeprecatedPaintLayer.h
diff --git a/Source/core/paint/DeprecatedPaintLayer.h b/Source/core/paint/DeprecatedPaintLayer.h
index 032cdf2cdc32e74828f97b9bcf3e0f9a023b7217..f997d76316ba810b8dd2830376e9790ed694453c 100644
--- a/Source/core/paint/DeprecatedPaintLayer.h
+++ b/Source/core/paint/DeprecatedPaintLayer.h
@@ -95,6 +95,12 @@ private:
 // hardware acceleration (through DeprecatedPaintLayerCompositor),
 // scrolling (through DeprecatedPaintLayerScrollableArea)
 // along with some optimizations are all handled by DeprecatedPaintLayer.
+//
+// The class is DEPRECATED, which means that we would like to remove it. The
+// reason for removal is that it has been a dumping ground for features for too
+// long and is the wrong level of abstraction, bearing no correspondence to any
+// CSS concept. Its associated objects and some of its feature need to be
+// migrated to LayoutObject (or the appropriate sub-class).
 class CORE_EXPORT DeprecatedPaintLayer {
     WTF_MAKE_NONCOPYABLE(DeprecatedPaintLayer);
 public: