[css-backgrounds-3] Fixed definition for where box shadows apply

Author: SebastianZ

css-backgrounds-3/Overview.bs

@@ -2513,10 +2513,9 @@ Layering, Layout, and Other Details</h4>
   and the [=inner shadows=] of an element are drawn immediately above the background of that element
   (below the borders and border image, if any).
 
-  <p>Drop shadows are only applied to the [=principal box=].
-  If that box has multiple [=fragments=], then all of them get shadows,
-  but shadows are only drawn where borders would also be drawn;
-  see 'box-decoration-break'.
+  <p>Unless otherwise specified, drop shadows are only applied to the [=principal box=].
+  If the affected box has multiple fragments,
+  the shadows are applied as specified in 'box-decoration-break'.
 
   <p>Shadows do not trigger scrolling or increase the size of the scrollable area.