[css-mixins-1][editorial] Improve abstract

Author: tabatkins

css-mixins-1/Overview.bs

@@ -9,7 +9,7 @@ ED: https://drafts.csswg.org/css-mixins/
 TR: https://www.w3.org/TR/css-mixins-1/
 Editor: Miriam E. Suzanne, Invited Expert, http://miriamsuzanne.com/contact, w3cid 117151
 Editor: Tab Atkins-Bittner, Google, http://xanthir.com/contact/, w3cid 42199
-Abstract: This module provides the ability to define custom functional notations.
+Abstract: This module defines the ability for authors to define custom functions, acting similar to <em>parametrized</em> custom properties. They can use the full power of CSS's values and conditional rules. It also defines an early form of a similar idea for CSS rule mixins, allowing parametrized substitution of entire blocks of properties into other rules.
 Default Highlight: css
 Ignored Terms: cssText
 </pre>