Allow `_doc` as a type name

[jpountz]

As of 7.0, types support will be gone, the index API will be either PUT index/_doc/id or POST index/_doc and hits will use _doc as a fake type in the response. In order to allow users to have a smoother transition to 7.0, we should allow _doc as a type in 6.x.

Relates to #15613

[morphers82]

I have been naming my type the same as index, ie: PUT index_name/index_name/id
i don't think it should have to be anything specific or that will make many people's 6.0 indices not work with 7.0 if it has to be _doc.

[jpountz]

@morphers82 To clarify, this is not a requirement, this is just a way to potentially make the transition easier for users who decide to opt in for the type name that will be used in 7.0. 6.x indices will work in 7.0 regardless of the type name.

The benefit of switching to _doc earlier is that index APIs will work the same way on 6.x and 7.x indices: PUT index/_doc/id or POST index/_doc.