Update layout documentation re: coordinate systems and writing mode.

third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h

Index: third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h
diff --git a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h
index 5056ec97b002ea7b92fd4465db45dfc14781a35f..86c4b4c2035fa0814d6177a87004c9dfc988017a 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h
+++ b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h
@@ -127,8 +127,8 @@ public:
 // - physical coordinates with flipped block-flow direction: those are physical
 //   coordinates but we flipped the block direction. See
 //   LayoutBox::noOverflowRect.
-//   TODO(jchaffraix): I don't fully understand why we need this coordinate
-//   system someone should fill in those details.
+//
+// For more, see Source/core/layout/README.md ### Coordinate Spaces.
 class CORE_EXPORT LayoutBoxModelObject : public LayoutObject {
 public:
     LayoutBoxModelObject(ContainerNode*);