[selectors-4] Clarifications to complex selector matching. #2240 p=glazou, loiriol

Author: fantasai

selectors-4/Overview.bs

@@ -608,8 +608,9 @@ Structure and Terminology</h3>
 	A given element is said to <a>match</a> a <a>complex selector</a>
 	when there exists a list of elements,
 	each matching a corresponding <a>compound selector</a> in the <a>complex selector</a>,
-	with their relationships matching the <a>combinators</a> between them,
-	and with the given element matching the last <a>compound selector</a>.
+	with each pair of elements consecutive in the list matching
+	the <a>combinator</a> between their corresponding <a>compound selectors</a>,
+	and with the last element being the given element.
 
 	Note: Thus, a selector consisting of a single <a>compound selector</a>
 	matches any element satisfying the requirements