Update datehistogram-aggregation.asciidoc

Docs really need to explain how to suppress the hits portion of the response to shrink it down and speed it up. I had to do a lot of digging to figure out how and actual found it based on how to do it in facets. Although this probably effects all the various bucket aggregations. Perhaps the change should be made somewhere else in the docs. But made it here since this was where I needed it and tested it.

Hope this is helpful. Thanks!