[css-rhythm] Add block-level boxes to intro.

Author: fantasai

css-rhythm/Overview.bs

@@ -13,6 +13,7 @@ Group: CSSWG
 Status: ED
 Work Status: exploring
 Editor: Koji Ishii, Google, kojiishi@gmail.com
+Editor: Elika J. Etemad (fantasai), Invited Expert, http://fantasai.inkedblade.net/contact
 ED: https://drafts.csswg.org/css-rhythm/
 Abstract: This module contains CSS features for aligning content size
   to multiple of unit size.
@@ -33,7 +34,8 @@ Controlling sizes of CSS objects to be multiple of a unit
 is desired in many cases.
 This level of the specification focuses on following cases.
 
-* Adjust heights of line boxes to the multiple of the specified unit.
+* Adjust heights of line boxes to multiples of the specified length.
+* Adjust heights of block-level boxes to multiples of the specified length.
 
 By controlling heights of line boxes,
 lines of text in different fonts can create consistent visuals