[css-syntax-3] Catch infinite-loop bug.

Author: tabatkins

css-syntax-3/Overview.bs

@@ -2571,6 +2571,7 @@ Consume a qualified rule</h4>
 		<dt><<semicolon-token>>
 		<dd>
 			This is a <a>parse error</a>.
+			[=Discard a token=] from |input|.
 			Return nothing.
 
 		<dt><a href="#tokendef-close-curly">&lt;}-token></a>