[css-color] Disambiguate ''auto'' as ''z-index/auto''.

Author: dbaron

css-color/Overview.bs

@@ -2155,7 +2155,7 @@ Transparency: the 'opacity' property</h2>
 	This means that any content outside of it cannot be layered in z-order between pieces of content inside of it,
 	and vice versa.
 	If the element is in a context where the 'z-index' property applies,
-	the ''auto'' value is treated as ''0'' for the element.
+	the ''z-index/auto'' value is treated as ''0'' for the element.
 	See <a href="http://www.w3.org/TR/CSS21/visuren.html#layers">section 9.9</a>
 	and <a href="http://www.w3.org/TR/CSS21/zindex.html">Appendix E</a> of [[!CSS21]]
 	for more information on stacking contexts.