[css-overscroll-1] Clarify the wording to be about scrollable axis.

Author: tabatkins

css-overscroll-1/Overview.bs

@@ -142,9 +142,9 @@ reaches the edge of the <a>scrollport</a>.
 If a [=scroll container=] has no potential to scroll,
 because it does not <a>overflow</a> in the direction of the scroll,
 the element is always considered to be at the scroll boundary.
-If a [=scroll container=] is not scrollable in the given axis,
-it doesn't have a [=scroll boundary=] at all in that axis
-(and won't be part of this scroll's [=scroll chain=].)
+A [=single-axis scroll container=] only has a [=scroll boundary=] in its [=scrollable axis=];
+if the scroll is in its other axis,
+this element won't be part of the [=scroll chain=] at all.
 
 <dfn>Boundary default action</dfn> refers to the user-agent-defined <a>default action</a>
 performed when scrolling against the edge of the <a>scrollport</a>.