[css-variables] Per WG resolution, reserve -- for future use by CSS. Fixes w3c#2518.

Author: tabatkins

css-variables-1/Overview.bs

@@ -72,7 +72,9 @@ Defining Custom Properties: the '--*' family of properties</h2>
 	whose name starts with two dashes (U+002D HYPHEN-MINUS),
 	like '--foo'.
 	The <dfn>&lt;custom-property-name></dfn> production corresponds to this:
-	it's defined as any valid <a>identifier</a> that starts with two dashes.
+	it's defined as any valid <a>identifier</a> that starts with two dashes,
+	except ''--'' itself,
+	which is reserved for future use by CSS.
 	<a>Custom properties</a> are solely for use by authors and users;
 	CSS will never give them a meaning beyond what is presented here.