[selector] Use infra terminology for ascii case insentivity (#13427)

See #4154 (comment)

Author: frivoal

selectors-4/Overview.bs

@@ -2619,7 +2619,7 @@ The Language Pseudo-class: '':lang()''</h3>
 	must be <i>canonicalized</i>
 	and converted to <i>extlang form</i> as per section 4.5 of [[!RFC5646]]
 	prior to the <i>extended filtering</i> operation.
-	The matching is performed case-insensitively within the ASCII range.
+	The matching is performed [=ASCII case-insensitively=].
 
 	The <a>language range</a> does not need to be a valid language code to
 	perform this comparison.