make abstract self-contained (w3c#1229)

Author: dret

css-images-4/Overview.bs

@@ -13,7 +13,7 @@ Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.n
 Editor: Lea Verou, Invited Expert, http://lea.verou.me/about, w3cid 52258
 Abstract: This module contains the features of CSS level 4 relating to the <<image>> type and replaced elements.
 	It includes and extends the functionality of CSS level 2 [[CSS21]] and in the previous level of this specification [[css3-images]].
-	The main extensions compared to [[css3-images]] are several additions to the <<image>> type, such as the ''image()'' notation, the ''element()'' notation, and conic gradients.
+	The main extensions compared to "CSS Image Values and Replaced Content Module Level 3" [[css3-images]] are several additions to the <<image>> type, such as the ''image()'' notation, the ''element()'' notation, and conic gradients.
 
 Issue Tracking: Tracker http://www.w3.org/Style/CSS/Tracker/products/27
 Previous Version: https://www.w3.org/TR/2012/WD-css4-images-20120911/