add more clarification about special background painting area for root

Author: fantasai

css3-background/Overview.src.html

@@ -832,8 +832,9 @@ <h3 id="the-background-clip">The 'background-clip' property</h3>
 </dl>
 
 <p class=note>Note that the root element has a different background
-painting area. See <a href="#special-backgrounds"> “The backgrounds
-of special elements.”</a>
+painting area, and thus the 'background-clip' property has no effect
+when specified on it. See <a href="#special-backgrounds"> “The
+backgrounds of special elements.”</a>
 
 <p class=note>Note that the background is always drawn <em>behind</em>
 the border, if any. See “Elaborate description of Stacking Contexts” in