[css-navigation-1] Clarify name matching of descriptors to URLPatternInit keys.

Followup to w3c#13177.

Author: dbaron

css-navigation-1/Overview.bs

@@ -73,8 +73,12 @@ The ''@route'' rule can be defined in one of two ways:
 :: In this case the URL pattern represented is the result of invoking
 	[=URL pattern/create|create a URL pattern=] given
 	<var>input</var> as {{URLPatternInit}}
-	constructed from the descriptors and their values;
-	if a <code>base-url</code> descriptor is not given then one is created from
+	constructed from the descriptors and their values.
+	Each dictionary member is given the value of
+	the descriptor with the same name,
+	except the <code>baseURL</code> member is given the value of
+	the <code>base-url</code> descriptor.
+	If a <code>base-url</code> descriptor is not given then one is created from
 	the [=style resource base URL=] of the rule.
 
 ISSUE: Should this use <<dashed-ident>>, <<custom-ident>>, or <<ident>>