[css-nesting-1] Add issue about lookahead per WG resolution

https://lists.w3.org/Archives/Public/www-style/2023Jan/0012.html

Author: fantasai

css-nesting-1/Overview.bs

@@ -274,6 +274,9 @@ Syntax {#syntax}
 	The precise details of how nested style rules are parsed
 	are defined in [[!CSS-SYNTAX-3]].
 
+	ISSUE(7961): The CSSWG is currently exploring the consequences of parsing lookahead,
+	and may adjust the allowed syntax as a result.
+
 	An invalid [=nested style rule=] is ignored,
 	along with its contents,
 	but does not invalidate its parent rule.