[selectors-4] Reword constraints on language ranges, as suggested by reviewer; see w3c#8720

Co-authored by: Addison Phillips <addisonI18N@gmail.com>

Author: svgeesus

selectors-4/Overview.bs

@@ -2640,10 +2640,14 @@ The Language Pseudo-class: '':lang()''</h3>
 	prior to the <i>extended filtering</i> operation.
 	The matching is performed [=ASCII case-insensitively=].
 
-	The <a>language range</a> 
-	must be <em>well-formed</em> according to the syntax of BCP 47 in order to match.
-	However, it does not need to be a <em>valid</em> language code to
-	perform this comparison.
+	The <a>language range</a>
+	must be an extended language range according to BCP47.
+	Language ranges that are not
+	<a href="https://www.w3.org/TR/i18n-glossary/#dfn-well-formed">well-formed</a> language tags
+	or which would not be a well-formed language tag 
+	if an initial wildcard character "*" 
+	were replaced with a valid subtag,
+	do not match anything.
 
 	<div class="example">
 		For example,