[css-ui-4] url-set/image-set are not recursive

Addresses #8030 and part of #7388

css-ui-4/Overview.bs

@@ -742,8 +742,7 @@ Styling the Cursor: the 'cursor' property</h4>
 					support <<image>> which is a superset.
 
 					<dfn type>&lt;url-set></dfn> is a limited version of ''image-set()'',
-					where only <<url>> and <<url-set>> can be used
-					in the part of the <<image-set-option>> sub-production which would normally allow <<image>>.
+					where the <<image>> sub-production is restricted to <<url>> only.
 
 					The UA must support the following image file formats: