[css-overflow-3] Improve grammar by splitting one sentence into two. (#7277)

Co-authored-by: L. David Baron <dbaron@dbaron.org>

Author: dlrobertson

css-overflow-3/Overview.bs

@@ -589,11 +589,12 @@ Scrollbars and Layout</h3>
 <h3 id="scrolling-direction">
 Scrolling Origin, Direction, and Restriction</h3>
 
-	The <dfn export>initial scroll position</dfn>,
-	that is, the initial position of
+	The <dfn export>initial scroll position</dfn> is
+	the initial position of
 	the box’s <a>scrollable overflow area</a>
 	with respect to its border box,
-	prior to any user or programmatic scrolling that changes it,
+	prior to any user or programmatic scrolling that changes it.
+	The <a>initial scroll position</a>
 	is dependent on the box’s <a>writing mode</a>,
 	and is by default the <a>block-start</a>/<a>inline-start</a> edge
 	of the box’s <a>padding edge</a>.