Add documentation for disabling _field_names meta-field

[inqueue]

The _field_names meta-field can be disabled just like _all. For example:

PUT my_index
{
  "mappings": {
    "type_1": { 
      "properties": {...}
    },
    "type_2": { 
      "_field_names": {
        "enabled": false
      },
      "properties": {...}
    }
  }
}

Could documentation be provided for disabling _field_names along with the implications?

https://www.elastic.co/guide/en/elasticsearch/reference/5.6/mapping-field-names-field.html

@jpountz