[css-transitions] Fix ambiguous reference for 'background-image', and link to latest css2 rather than css21.

Author: dbaron

css-transitions/Overview.bs

@@ -46,7 +46,7 @@ Ignored Vars: x1, x2, y1, y2
 Link Defaults: css-transforms (property) transform
 </pre>
 <pre class="link-defaults">
-spec:css21; type:property;
+spec:css2; type:property;
     text:top
     text:right
     text:bottom
@@ -88,6 +88,9 @@ spec:css21; type:property;
     text:visibility
     text:word-spacing
     text:z-index
+spec:css-backgrounds-3; type:property;
+    text:background-image
+    text:background-origin
 spec:css-color-3;
     type:property;
         text:color