Document the ClientRect* vs DOMRect* situation

Source/core/dom/ClientRectList.cpp

 /*
  * Copyright (C) 2009 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 33 matching lines @@
         m_list.append(ClientRect::create(quads[i].boundingBox()));
 }
 
 unsigned ClientRectList::length() const
 {
     return m_list.size();
 }
 
 ClientRect* ClientRectList::item(unsigned index)
 {
-    if (index >= m_list.size()) {
-        // FIXME: this should throw an exception.
-        // ec = IndexSizeError;
+    if (index >= m_list.size())
         return 0;
-    }
 
     return m_list[index].get();
 }
 
 DEFINE_TRACE(ClientRectList)
 {
     visitor->trace(m_list);
 }
 
 } // namespace blink