[css-backgrounds-3] Rewrite corner clipping paragraph to hopefully be less confusing. w3c#5680

Author: fantasai

css-backgrounds-3/Overview.bs

@@ -1966,14 +1966,17 @@ in calculating the corner radii of the margin box shape.
 
 <h3 id="corner-clipping">Corner Clipping</h3>
 
-<p>A box's backgrounds, but not its <a href="#the-border-image">border-image</a>,
-are clipped to the appropriate curve (as determined by 'background-clip').
-Other effects that clip to the border or padding edge (such as 'overflow'
-other than ''visible'') also must clip to the curve. The content of
-replaced elements is always trimmed to the content edge curve.
-
-<p>Also, the area outside the curve of the border edge does not
-accept pointer events on behalf of the element.
+<p>Although <a href="#the-border-image">border images</a> are not affected
+by 'border-radius',
+other effects that clip painting or event handling
+to the border, padding, or content edge
+must clip to their respective curves.
+For example,
+backgrounds clip to the curve specified by 'background-clip',
+'overflow' values other than ''visible'' to the curved padding edge,
+[=replaced element=] content to the curved content edge,
+pointer events to the curved border edge,
+etc.
 
 <p class="note">As 'border-radius' reduces the interactive area of an element
 authors should make sure the remaining interactive area conforms