[css-sizing-4] Update abstract

Author: fantasai

css-sizing-4/Overview.bs

@@ -11,7 +11,7 @@ Editor: Tab Atkins, Google, http://xanthir.com/contact/, w3cid 42199
 Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
 Editor: Jen Simmons, Mozilla, http://jensimmons.com/
 Previous version: https://www.w3.org/TR/2012/WD-css3-sizing-20120927/
-Abstract: This is a delta spec over CSS Sizing Level 3.
+Abstract: This module extends the CSS sizing properties with keywords that represent content-based "intrinsic" sizes and context-based "extrinsic" sizes, allowing CSS to more easily describe boxes that fit their content or fit into a particular layout context. This is a delta spec over CSS Sizing Level 3.
 Ignored Terms: block-level box
 </pre>
 
@@ -36,7 +36,7 @@ spec:css2; type: property
 <h2 id="intro">
 Introduction</h2>
 
-	ISSUE: This is a diff spec from <a href="https://www.w3.org/TR/css-sizing-3/">CSS Sizing Level 3</a>.
+	ISSUE: This is a diff spec over <a href="https://www.w3.org/TR/css-sizing-3/">CSS Sizing Level 3</a>.
 	It is currently an Exploratory Working Draft:
 	if you are implementing anything, please use Level 3 as a reference.
 	We will merge the Level 3 text into this draft once it reaches CR.