[css3-sizing] Specify that fill-available is computed after margin-collapsing.

Author: tabatkins

css3-sizing/Overview.html

@@ -868,7 +868,9 @@ <h2 id=extrinsic-sizing><span class=secno>5. </span> Extrinsic Size
     </ul>
   </ul>
 
-  <p> …less the box's inline-axis margins, borders, and padding.
+  <p> …less the box's inline-axis margins (after <a
+   href="http://www.w3.org/TR/CSS21/box.html#collapsing-margins">margin
+   collapsing</a>), borders, and padding.
 
   <p> The <a href="#fill-available-extent"><i>fill-available extent</i></a>
    of a box is defined analogously, but in the other dimension.

css3-sizing/Overview.src.html

@@ -612,7 +612,7 @@ <h2 id='extrinsic-sizing'>
 				</ul>
 		</ul>
 
-		&hellip;less the box's inline-axis margins, borders, and padding.
+		&hellip;less the box's inline-axis margins (after <a href="http://www.w3.org/TR/CSS21/box.html#collapsing-margins">margin collapsing</a>), borders, and padding.
 
 	<p>
 		The <i>fill-available extent</i> of a box is defined analogously,