Bug 616884. Add comment to note that stored clip region isn't updated. a=comment

widget/src/windows/nsWindow.h

@@ -460,6 +460,12 @@ protected:
   void                    StopFlashing();
   static PRBool           IsTopLevelMouseExit(HWND aWnd);
   static void             SetupKeyModifiersSequence(nsTArray<KeyPair>* aArray, PRUint32 aModifiers);
+  /*
+   * If aIntersectWithExisting is true then the stored clip region isn't
+   * updated (only the actual clip on the window) so this should be called
+   * again soon afterward with aIntersectWithExisting false so the stored clip
+   * region does get updated.
+   */
   nsresult                SetWindowClipRegion(const nsTArray<nsIntRect>& aRects,
                                               PRBool aIntersectWithExisting);
   nsIntRegion             GetRegionToPaint(PRBool aForceFullRepaint,