[css-syntax] Expand the note in 'consume a url token' to make it clear that this algo doesn't consume quoted urls.

Author: tabatkins

css-syntax-3/Overview.bs

@@ -1203,6 +1203,12 @@ Consume a url token</h4>
 	It returns either a <<url-token>> or a <<bad-url-token>>.
 
 	Note: This algorithm assumes that the initial "url(" has already been consumed.
+	This algorithm also assumes that it's being called to consume an "unquoted" value,
+	like ''url(foo)''.
+	A quoted value, like ''url("foo")'',
+	is parsed as a <<function-token>>.
+	<a>Consume an ident-like token</a> automatically handles this distinction;
+	this algorithm shouldn't be called directly otherwise.
 
 	<ol>
 		<li>