[css-syntax] 'consume an at-rule' always returns a value, so no need for the caveat. Fixes w3c#1839.

Author: tabatkins

css-syntax-3/Overview.bs

@@ -2241,9 +2241,8 @@ Consume a list of rules</h4>
 		<dt><<at-keyword-token>>
 		<dd>
 			<a>Reconsume the current input token</a>.
-			<a>Consume an at-rule</a>.
-			If anything is returned,
-			append it to the list of rules.
+			<a>Consume an at-rule</a>,
+			and append the returned value to the list of rules.
 
 		<dt>anything else
 		<dd>