[selector-4][editorial] Add a note spelling out the interaction of relative-selector vs compound-selector restrictions in :has().

Author: tabatkins

selectors-4/Overview.bs

@@ -1269,6 +1269,12 @@ The Relational Pseudo-class: '':has()''</h3>
 	many of them exist conditionally, based on the styling of their ancestors,
 	so allowing these to be queried by '':has()'' would introduce cycles.
 
+	Note: Since '':has()'' takes a <<relative-selector-list>>,
+	its arguments are <em>inherently</em> [=complex selectors=]
+	(because they start, perhaps implicitly, with a combinator).
+	This means '':has()'' cannot be used in contexts that don't allow complex selectors;
+	its arguments will be guaranteed to be invalid.
+
 	<div class='example'>
 		For example, the following selector matches only
 		<code>&lt;a></code> elements that contain an <code>&lt;img></code> child: