title: why can't "_all" field be enabled in ElasticSearch 6.0?
body: <p>I have the index "index" and type "fulltext" created, but when I was trying to enable "_all" field with (as suggested by <a href="https://www.elastic.co/guide/en/elasticsearch/reference/6.0/mapping-all-field.html" rel="nofollow noreferrer">https://www.elastic.co/guide/en/elasticsearch/reference/6.0/mapping-all-field.html</a>):</p>
  
  <pre><code>curl -XPUT 'localhost:9200/index?pretty' -H 'Content-Type: application/json' -d'
  {
    "mapping": {
      "fulltext": {
        "_all": {
          "enabled": true   
        }
      }
    }
  }
  '
  </code></pre>
  
  <p>The message I got was as follows:</p>
  
  <pre><code>{
    "error" : {
      "root_cause" : [
        {
          "type" : "illegal_argument_exception",
          "reason" : "unknown setting [index.mapping.fulltext._all.enabled] please check that any required plugins are installed, or check the breaking changes documentation for removed settings"
        }
      ],
      "type" : "illegal_argument_exception",
      "reason" : "unknown setting [index.mapping.fulltext._all.enabled] please check that any required plugins are installed, or check the breaking changes documentation for removed settings"
    },
    "status" : 400
  }
  </code></pre>
  
  <p>How can I correct it? Thanks.</p>