[css-syntax] Add some Ignored Vars declarations.

Author: tabatkins

css-syntax/Overview.bs

@@ -15,6 +15,7 @@ Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/
 Editor: Simon Sapin, Mozilla, http://exyr.org/about/
 Abstract: This module describes, in general terms, the basic structure and syntax of CSS stylesheets. It defines, in detail, the syntax and parsing of CSS - how to turn a stream of bytes into a meaningful stylesheet.
 Ignored Terms: <keyframes-name>, <keyframe-rule>, <keyframe-selector>, <translation-value>, <media-query-list>, <unicode-range-token>
+Ignored Vars: +b, -b, foo
 Link Defaults: css-text-decor-3 (property) text-decoration, css-color-3 (property) color, css-transforms-1 (function) translatex()
 </pre>