title: "_doc" mapping type name not accepted in ElasticSearch 5.6
body: <p>I am looking at examples of single-type indices on ElasticSearch 5.6 to prepare for the removal of mapping types. Specifically, I am running the first example from the ElasticSearch <a href="https://www.elastic.co/guide/en/elasticsearch/reference/master/removal-of-types.html#_index_per_document_type_2" rel="noreferrer">page about the removal of types</a>, on a fresh cluster running locally in Docker using the <code>docker.elastic.co/elasticsearch/elasticsearch:5.6.5</code> image</p>
  
  <p>Running the first example from section I linked to:</p>
  
  <pre><code>PUT localhost:9200/users
  {
    "settings": {
      "index.mapping.single_type": true
    },
    "mappings": {
      "_doc": {
        "properties": {
          "name": {
            "type": "text"
          },
          "user_name": {
            "type": "keyword"
          },
          "email": {
            "type": "keyword"
          }
        }
      }
    }
  }
  </code></pre>
  
  <p>I get the following error:</p>
  
  <pre><code>{
    "error": {
      "root_cause": [
        {
          "type": "invalid_type_name_exception",
          "reason": "mapping type name [_doc] can't start with '_'"
        }
      ],
      "type": "invalid_type_name_exception",
      "reason": "mapping type name [_doc] can't start with '_'"
    },
    "status": 400
  }
  </code></pre>
  
  <p>I understand that fields with a leading underscore in the name are generally considered as reserved for ES internals; but I was assuming that <code>_doc</code> would be considered a special case starting with version <code>5.6</code>, since the linked guide mentions:</p>
  
  <blockquote>
    <p>Indices created in 6.x only allow a single-type per index. Any name can be used for the type, but there can be only one. The preferred type name is _doc so that index APIs have the same path as they will have in 7.0</p>
  </blockquote>
  
  <p>Am I missing something, such as a cluster setting?</p>