[css-values-4] Linkify 'case-sensitive' to make it clearer.

Author: tabatkins

css-values-4/Overview.bs

@@ -398,7 +398,8 @@ Author-defined Identifiers: the <<custom-ident>> type</h3>
 	This generic data type is denoted by <dfn id="identifier-value">&lt;custom-ident></dfn>,
 	and represents any valid CSS <a>identifier</a>
 	that would not be misinterpreted as a pre-defined keyword in that property's value definition.
-	Such identifiers are fully case-sensitive,
+	Such identifiers are fully [=case-sensitive=]
+	(meaning they're compared by codepoint),
 	even in the ASCII range
 	(e.g. ''example'' and ''EXAMPLE'' are two different, unrelated user-defined identifiers).