[css-flexbox-1] Revert erroneous simplification. w3c#5179 (comment)

Author: fantasai

css-flexbox-1/Overview.bs

@@ -2817,8 +2817,10 @@ Resolving Flexible Lengths</h3>
 
 		<li>
 			<strong>Calculate <dfn>initial free space</dfn>.</strong>
-			Sum the outer [=target main sizes=] of all items on the line,
+			Sum the outer sizes of all items on the line,
 			and subtract this from the flex container's inner <a>main size</a>.
+			For frozen items, use their outer <a>target main size</a>;
+			for other items, use their outer <a>flex base size</a>.
 
 		<li>
 			Loop: