Deprecated in 6.0.0.See Removal of mapping types
The default mapping, which will be used as the base mapping for a new
mapping type, can be customised by adding a mapping type with the name
_default_ to an index, either when
creating the index or later on with the
PUT mapping API.
The documentation for this feature has been removed as it no longer makes sense in 6.x where there can be only a single type per index.