[css-highlight-api-1] Add note recommending authors to use custom highlight names that are CSS identifiers for easier styling via CSS.

Author: sanketj

css-highlight-api-1/Overview.bs

@@ -229,6 +229,10 @@ Registering Custom Highlights</h3>
 		The <dfn>custom highlight name</dfn> assigned to a [=custom highlight=] when it is [=registered=]
 		is used to identify the highlight during styling (see [[#styling-highlights]]).
 
+	Note: When registering a [=custom highlight=], authors are recommended to use a
+	[=custom highlight name=] that is a valid CSS [=identifier=]. Using a name that is not a valid
+	identifier can make the highlight hard, and in some cases impossible, to style via CSS.
+
 <h2 id=styling-highlights>
 Styling Custom Highlights</h2>