Adds URL for preview data frame transforms

x-pack/plugin/src/test/resources/rest-api-spec/api/data_frame.preview_data_frame_transform.json

@@ -1,6 +1,6 @@
 {
   "data_frame.preview_data_frame_transform": {
-    "documentation": "TODO",
+    "documentation": "https://www.elastic.co/guide/en/elasticsearch/reference/current/preview-data-frame-transform.html",
     "methods": [ "POST" ],
     "url": {
       "path": "/_data_frame/transforms/_preview",