[cssom-1] Make it clearer that the CSSRule.type enumeration is frozen. #7154

Author: tabatkins

cssom-1/Overview.bs

@@ -1990,6 +1990,9 @@ is a legacy design practice that is no longer used in Web APIs.
 Instead, to tell what type of rule a given object is,
 it is recommended to check <code highlight=js>rule.constructor.name</code>,
 which will return a string like <code highlight=js>"CSSStyleRule"</code>.
+This enumeration is thus frozen in its current state,
+and no new new values will be added to reflect additional at-rules;
+all at-rules beyond the ones listed above will return 0.
 
 
 ### The {{CSSStyleRule}} Interface ### {#the-cssstylerule-interface}