title: what does _doc represents in elasticsearch?
body: <p>I am still trying to figure out what does _doc represents in elasticsearch?
  From the documentation, two places I could find it's usage are:</p>
  
  <ol>
  <li><p>While in sorting, it is recommended that _doc is better because the benefit of sorting by _doc is that elasticsearch can efficiently skip to the next matching document when moving to the next page (it will simply ignore all docs that have a smaller doc id than the last returned document).  <a href="https://github.com/elastic/elasticsearch/pull/12983#issuecomment-181118167" rel="noreferrer">Source</a></p></li>
  <li><p>Another reference to _doc was mentioned in <a href="https://github.com/elastic/elasticsearch/issues/4492" rel="noreferrer">this</a> git request which talks putting field name against _doc.</p></li>
  </ol>
  
  <p>Can someone exactly tell what is _doc actually?</p>