[css-highlight-api-1] Clarifying that user agents must not use live ranges internally for static ranges w3c#4597 (w3c#6545)

* Add note clarifying that user agents must not use live ranges internally for static ranges

* Use spec reference syntax

* Pull the note into the normative paragraph.

Author: dandclark

css-highlight-api-1/Overview.bs

@@ -540,6 +540,8 @@ Range Updating and Invalidation</h3>
 	the user agent must not adjust the [=boundary points=] of {{StaticRange}}s
 	in response to DOM changes,
 	nor can they be modified by the author after creation.
+	The user agent is expected to store the actual {{StaticRange}}s, rather than backing
+	them up with live {{Range}}s.
 
 	<div class=advisement>
 		Updating all {{Range}} objects as the DOM is modified