[css-align-3] Reword scrolling-alignment to hopefully make it more clear. Related to #1425.

Author: tabatkins

css-align-3/Overview.bs

@@ -1073,6 +1073,10 @@ Overflow and Scroll Positions</h3>
 	of the <a>scroll container</a>
 	satisfies the expected alignment of the <a>alignment subject</a> and <a>alignment container</a>.
 
+	Note: The presence of scrollbars can change the size of the [=scroll container’s=] content box,
+	and thus the size of the [=alignment container=]
+	and/or [=alignment subject=].
+
 	<div class='example'>
 		For example,
 		if a scrollable flex container is set to ''justify-content: flex-end''
@@ -1081,18 +1085,22 @@ Overflow and Scroll Positions</h3>
 		and its content will appear to overflow its main-start edge.
 	</div>
 
-	Performing such alignment can require that the <a>scrollable overflow region</a>
-	be extended by the amount of padding specified
-	on the end edges of the <a>scroll container</a>.
-	Therefore,
-	for consistency,
-	any non-''align-content/normal'' value of 'align-content' or 'justify-content'
-	causes the <a>scrollable overflow region</a> to be extended
+	Because the <a>scroll container</a>’s end-edge padding
+	is not always included in the <a>scrollable overflow region</a>
+	(see [[css-overflow-3#scrollable]]),
+	performing such alignment might require
+	that the <a>alignment subject</a> be positioned
+	in a way that would normally not be possible just by scrolling.
+	To resolve this problem
+	(and be consistent across alignment values),
+	all non-''align-content/normal'' values of 'align-content'/'justify-content'
+	cause the <a>scrollable overflow region</a> to be extended
 	in the relevant axis
-	however much would be required to satisfy ''align-content/end'' alignment
-	in that axis.
+	however much would be required to make it possible to scroll
+	to a position satisfying ''align-content/end'' alignment in that axis.
 	(In general, this means that the <a>scroll container</a>’s padding
-	is added to the bounding rectangle of its in-flow children.
+	is added to the bounding rectangle of its in-flow children
+	on <em>both</em> sides of the <a>scrollable overflow region</a>.
 	See [[css-overflow-3]] for details.)
 
 	ISSUE: Coordinate wording with css-overflow once it's less of a mess.