What metafields does storedFields("_none_") return?

Can someone explain what metafield does storedFields("_none") return? As per the javadoc

Adds stored fields to load and return (note, it must be stored) as part of the search request.
To disable the stored fields entirely (source and metadata fields) use {@code storedField("none")}.

But whenever I am sending a request like below

SearchRequestBuilder request;
...
request.setSize(config.getScanScrollFetchSize());
request.addSort(FieldSortBuilder.DOC_FIELD_NAME, SortOrder.ASC);
request.setScroll(TimeValue.timeValueMinutes(1));
request.storedFields("_none_");

It is giving me the response like below

{
  "_scroll_id": "DnF1ZXJ5VGhlbkZldGNoBQAAAAAAAAABFjMtc3Y1Rm5PU2pDVi1RM29FVzQyQmcAAAAAAAAAAxYzLXN2NUZuT1NqQ1YtUTNvRVc0MkJnAAAAAAAAAAIWMy1zdjVGbk9TakNWLVEzb0VXNDJCZwAAAAAAAAAEFjMtc3Y1Rm5PU2pDVi1RM29FVzQyQmcAAAAAAAAABRYzLXN2NUZuT1NqQ1YtUTNvRVc0MkJn",
  "took": 126,
  "timed_out": false,
  "_shards": {
    "total": 5,
    "successful": 5,
    "skipped": 0,
    "failed": 0
  },
  "hits": {
    "total": 1,
    "max_score": null,
    "hits": [
      {
        "_index": "denorm",
        "_score": null,
        "sort": [
          2
        ]
      }
    ]
  }
}

As per the doc _index is also a meta field but I am seeing this in the response.

Can someone let me know what all meta fields are returned when I use storedField("_none_") ?

The same has been asked in stackoverflow also. But no response there also. Anyone any thoughts on this?