[css-2015] Add definition of prefixed syntax.

Author: fantasai

css-2015/Overview.bs

@@ -434,6 +434,9 @@ Proprietary and Non-standardized Features</h4>
 
 	Issue: Rewritten as open vs. closed systems instead of Web vs. non-Web, per szilles' request.
 
+	To avoid clashes with future CSS features, the CSS2.1 specification
+	reserves a <a href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
+	syntax</a> for proprietary and experimental extensions to CSS.
 	A CSS feature is a <dfn>proprietary extension</dfn> if it is meant for use
 	in a closed environment accessible only to a single vendor's user agent(s).
 	A UA should support such <a>proprietary extensions</a>