"_doc" mapping type name not accepted in ElasticSearch 5.6

Question

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 page about the removal of types, on a fresh cluster running locally in Docker using the docker.elastic.co/elasticsearch/elasticsearch:5.6.5 image

Running the first example from section I linked to:

PUT localhost:9200/users
{
  "settings": {
    "index.mapping.single_type": true
  },
  "mappings": {
    "_doc": {
      "properties": {
        "name": {
          "type": "text"
        },
        "user_name": {
          "type": "keyword"
        },
        "email": {
          "type": "keyword"
        }
      }
    }
  }
}

I get the following error:

{
  "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
}

I understand that fields with a leading underscore in the name are generally considered as reserved for ES internals; but I was assuming that _doc would be considered a special case starting with version 5.6, since the linked guide mentions:

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

Am I missing something, such as a cluster setting?

Answer 1

The document I linked to is the master version. In the 6.1 or 5.6 versions of that same document, there is no mention of _doc being the preferred name; which likely means that the ability to use _doc as a mapping type name will come with future 6.x versions.