[css-scroll-snap] Make the order of operations clearer. Fixes #2214.

Author: tabatkins

css-scroll-snap-1/Overview.bs

@@ -508,9 +508,10 @@ Scroll Snapping Area: the 'scroll-margin' property {#scroll-margin}
     Values represent outsets defining the
     <dfn lt="scroll snap area" local-lt="snap area">scroll snap area</dfn>
     that is used for snapping this box to the snapport.
-    The <a>scroll snap area</a> is the rectangular bounding box of the transformed border box,
-    plus the specified outsets,
-    axis-aligned in the <a>scroll container’s</a> coordinate space.
+    The <a>scroll snap area</a> is determined by taking the transformed border box,
+    finding its rectangular bounding box
+    (axis-aligned in the <a>scroll container’s</a> coordinate space),
+    then adding the specified outsets.
 
     Note: This ensures that the <a>scroll snap area</a> is always rectangular
     and axis-aligned to the <a>scroll container’s</a> coordinate space.