[css-backgrounds-3] Cross link to 'ink overflow' terminology

(This terminology was introduced after css-backgrounds-3 was written.)

css-backgrounds-3/Overview.bs

@@ -2268,6 +2268,7 @@ Edge Overhang: the 'border-image-outset' property</h3>
  Negative values are invalid.
 
  Portions of the border-image that are rendered outside the [=border box=]
+ are [=ink overflow=] and
  do not trigger scrolling.
  Also such portions are invisible to mouse events
  and do not capture such events on behalf of the element.