[css-ui-4] Add full <image> support for 'cursor' property

Author: SebastianZ

css-ui-4/Overview.bs

@@ -586,7 +586,7 @@ user resizing of that element.
 
 <pre class="propdef">
 Name: cursor
-Value: [ [<<url>> [&lt;x&gt; &lt;y&gt;]?,]* <br>
+Value: [ [<<image>> [&lt;x&gt; &lt;y&gt;]?,]* <br>
  [ auto | default | none |<br>
  context-menu | help | pointer | progress | wait | <br>
  cell | crosshair | text | vertical-text | <br>
@@ -648,7 +648,6 @@ Values have the following meanings:
 			it must attempt to handle the second, etc.
 			If the user agent cannot handle any user-defined cursor,
 			it must use the cursor keyword at the end of the list.
-			Conforming User Agents may, instead of <<url>>, support <<image>> which is a superset.
 
 			The UA must support the following image file formats: