[css-backgrounds-4] Defined interaction of border-area and text values for background-clip (#12739)

Author: SebastianZ

css-backgrounds-4/Overview.bs

@@ -351,6 +351,9 @@ Painting Area: the 'background-clip' property</h3>
 		</dd>
 	</dl>
 
+	If both ''background-clip/border-area'' and ''background-clip/text'' are specified,
+	the background is painted within (clipped to) the union of these two areas.
+
 <h3 id='background-layers'>
 Background Image Layers: the 'background-tbd' shorthand property</h3>