[css-values-4] Adjust headings for <custom-ident> and <dashed-ident> to be more parallel.

Author: fantasai

css-values-4/Overview.bs

@@ -639,7 +639,7 @@ CSS-wide keywords: ''initial'', ''inherit'' and ''unset''</h4>
 	<!-- Make it easier to add CSS-wide keywords by defining a grammar production. -->
 
 <h3 id='custom-idents'>
-Author-defined Identifiers: the <<custom-ident>> type</h3>
+Unprefixed Author-defined Identifiers: the <<custom-ident>> type</h3>
 
 	Some properties accept arbitrary author-defined identifiers as a component value.
 	This generic data type is denoted by <dfn id="identifier-value">&lt;custom-ident></dfn>,
@@ -670,11 +670,12 @@ Author-defined Identifiers: the <<custom-ident>> type</h3>
 	</div>
 
 	Note: When designing grammars with <<custom-ident>>,
-	the <<custom-ident>> should always be "positionally unambiguous",
+	the <<custom-ident>> should always be “positionally unambiguous”,
 	so that it's impossible to conflict with any keyword values in the property.
+	Such conflicts can alternatively be avoided by using <<dashed-ident>>.
 
 <h3 id='dashed-idents'>
-<em>Explicitly</em> Author-defined Identifiers: the <<dashed-ident>> type</h3>
+Prefixed Author-defined Identifiers: the <<dashed-ident>> type</h3>
 
 	Some contexts accept <em>both</em> author-defined identifiers
 	<em>and</em> CSS-defined identifiers.