Amend the overly-large elements section to handle nested scroll areas.

Author: tabatkins

css-scroll-snap/Overview.bs

@@ -605,6 +605,7 @@ Scroll Snapping Alignment: the 'scroll-snap-align' property {#scroll-snap-alignm
 	correspond to the x and y axes, and not the y and x axes, respectively.
 
 	If the <a>snap area</a> is larger than the <a>snap viewport</a> in a particular axis,
+	and there are no other <a>snap areas</a> within the <a>snap viewport</a>,
 	then any scroll position in which the <a>snap area</a> covers the <a>snap viewport</a>
 	is a valid <a>snap position</a> in that axis.
 	The UA may use the specified alignment as a more precise target