[css-sizing-3] Simplify the behavior around fit-content(%) to just defer to normal % behavior, as suggested by Oriol in #3731.

Author: tabatkins

css-sizing-3/Overview.bs

@@ -468,15 +468,11 @@ Sizing Values: the <<length-percentage>>, ''width/auto'' | ''max-width/none'', '
 			Use the fit-content formula
 			with the <a>available space</a> replaced by the specified argument,
 			i.e.
-			<code>min(''width/max-content'', max(''width/min-content'', <<length-percentage>>))</code>.
+			<code>min(''width/max-content'', max(''width/min-content'', <<length-percentage>>))</code>,
+            where the <<length-percentage>> argument is resolved
+            exactly as for <<length-percentage>> values standing alone.
 
-			For <a>intrinsic size contributions</a>,
-			''fit-content(<<length>>)'' behaves as the resulting length;
-			''width/fit-content()'' containing a <<percentage>> behaves
-			as ''width/min-content'' for <a>min-content contributions</a>,
-			and as ''width/max-content'' for <a>max-content contributions</a>.
-
-			Negative values are invalid.
+			Negative <<length-percentage>> values are invalid.
 	</dl>
 
 	In all cases,