make it clear that hypothetical cross size is determined by performing layout

Author: tabatkins

css3-flexbox/Overview.html

@@ -2995,9 +2995,9 @@ <h3 id=main-sizing><span class=secno>9.3. </span> Main Size Determination</h3>
 
    <li id=algo-flex> <strong><a href="#resolve-flexible-lengths">Resolve the
     flexible lengths</a></strong> of all the flex items to find their used
-    main size, and <strong>determine their <dfn
-    id=hypothetical-cross-size>hypothetical cross size</dfn></strong>
-    assuming this main size. (See section 9.7.)
+    main size (see section 9.7.), and <strong>determine their <dfn
+    id=hypothetical-cross-size>hypothetical cross size</dfn></strong> by
+    performing layout assuming this used main size and the available space.
   </ol>
 
   <h3 id=cross-sizing><span class=secno>9.4. </span> Cross size determination</h3>

css3-flexbox/Overview.src.html

@@ -2312,8 +2312,10 @@ <h3 id='main-sizing'>
 			</ul>
 
 		<li id='algo-flex'>
-			<strong><a href='#resolve-flexible-lengths'>Resolve the flexible lengths</a></strong> of all the flex items to find their used main size,
-			and <strong>determine their <dfn>hypothetical cross size</dfn></strong> assuming this main size.  (See section 9.7.)
+			<strong><a href='#resolve-flexible-lengths'>Resolve the flexible lengths</a></strong> of all the flex items 
+			to find their used main size (see section 9.7.),
+			and <strong>determine their <dfn>hypothetical cross size</dfn></strong> 
+			by performing layout assuming this used main size and the available space.  
 	</ol>
 
 <h3 id='cross-sizing'>