The _index field mapping does not accept enabled

[clintongormley]

According to #9870, the _index field mapping should accept the enabled parameter, but it throws an exception:

PUT trial
{
  "mappings": {
    "doc": {
      "_index": {
        "enabled": true
      }
    }
  }
}

Returns:

{
   "error": {
      "root_cause": [
         {
            "type": "mapper_parsing_exception",
            "reason": "Mapping definition for [_index] has unsupported parameters:  [enabled : true]"
         }
      ],
      "type": "mapper_parsing_exception",
      "reason": "mapping [doc]",
      "caused_by": {
         "type": "mapper_parsing_exception",
         "reason": "Mapping definition for [_index] has unsupported parameters:  [enabled : true]"
      }
   },
   "status": 400
}

[rjernst]

We removed this setting, as it is no longer necessary: #12329.

I think we just need to update the docs?

[clintongormley]

@rjernst I'd completely forgotten about that. Thanks for the reminder. I've updated the docs accordingly