[css-lists-3] Counters inherit from both parent and sibling now, update intro to match.

Author: fantasai

css-lists-3/Overview.bs

@@ -991,8 +991,7 @@ Inheriting Counters</h4>
 
 	<div algorithm>
 		An element [=inherit counters|inherits=] its initial set of counters
-		from its preceding <em>sibling</em>,
-		or from its parent if it has none.
+		from its parent and preceding <em>sibling</em>.
 		It then takes the values for those counters
 		from the values of the matching counters
 		on its preceding <em>element in [=tree order=]</em>