[css-sizing-3] Clarify behaves as zero = used value of zero.

Author: fantasai

css-sizing-3/Overview.bs

@@ -371,7 +371,7 @@ Sizing Values: the <<length-percentage>>, ''width/auto'' | ''max-width/none'', '
 			For 'min-width'/'min-height',
 			specifies an <dfn dfn for>automatic minimum size</dfn>.
 			Unless otherwise defined by the relevant layout module,
-			this behaves as zero.
+			this behaves as (has a used value of) zero.
 
 			For backwards-compatibility,
 			the <a>resolved value</a> of this keyword is zero