[selectors-4] Explicitly link to the CSS/parse algorithm, as suggested in w3c#4927

Author: tabatkins

selectors-4/Overview.bs

@@ -3499,8 +3499,8 @@ Parse A Selector</h3>
 
 	<ol>
 		<li>
-			Let <var>selector</var> be the result of parsing <var>source</var> as a <<selector-list>>.
-			If it does not match the grammar,
+			Let <var>selector</var> be the result of [=CSS/parsing=] <var>source</var> as a <<selector-list>>.
+			If this returns failure,
 			it's an [=invalid selector=];
 			return failure.
 
@@ -3524,8 +3524,8 @@ Parse A Relative Selector</h3>
 
 	<ol>
 		<li>
-			Let <var>selector</var> be the result of parsing <var>source</var> as a <<relative-selector-list>>.
-			If it does not match the grammar,
+			Let <var>selector</var> be the result of [=CSS/parsing=] <var>source</var> as a <<relative-selector-list>>.
+			If this return failure,
 			return failure.
 
 		<li>