[css-lists-3] Mark counters section as deferring to CSS2 since it's still a bit shaky.

Author: fantasai

css-lists-3/Overview.bs

@@ -648,6 +648,10 @@ Automatic Numbering With Counters</h2>
 	a creator element,
 	and possibly another counter nested inside themselves.
 
+	ISSUE: This section is not fully synchronized with CSS2.
+	Please reference <a href="https://www.w3.org/TR/CSS2/generate.html#counters">Automatic Counters and Numbering in CSS Level 2</a>:
+	any conflicts or omissions here should be resolved in favor of the behavior specified in CSS2.
+
 <h3 id='counter-properties'>
 Manipulating Counters: the 'counter-increment', 'counter-set' and 'counter-reset' properties</h2>