[css-align] Make the interaction of content distribution properties and scroll containers clearer per w3c#1425

Author: fantasai

css-align/Overview.bs

@@ -865,14 +865,14 @@ Baseline Content-Alignment</h3>
 <h3 id="overflow-scroll-position">
 Overflow and Scroll Positions</h3>
 
-	The <a>content distribution properties</a> also affect the initial scroll position,
-	setting it to display the appropriate portion of the scrollable area.
-	In other words,
-	the <a>scrollable overflow region</a> is aligned relative to the viewport
-	as specified by the <a>content distribution properties</a>.
-	Additionally, so that the content remains in the <a>scrollable overflow region</a> in order to be scrollable,
-	the ''overflow/scroll'' and ''overflow/auto'' values of 'overflow'
-	trigger ''safe'' content alignment always.
+	Content that overflows a <a>scroll container</a>
+	is aligned exactly like content in a non-scrollable container.
+	However, to prevent content from being shifted into the unscrollable region,
+	when the <a>content distribution properties</a> are set on a <a>scroll container</a>,
+	the zero coordinate of its <a>scrollable overflow region</a>
+	(that is, the point beyond which content is unreachable)
+	is set to the top left corner of the <a>alignment subject</a>,
+	effectively changing the <a>initial scroll position</a>.
 
 	<div class='example'>
 		For example,