Allowing `_doc` as a type will enable users to make the transition to 7.0
smoother since the index APIs will be `PUT index/_doc/id` and `POST index/_doc`.
This also moves most of the documentation to `_doc` as a type name.
Closes#27750Closes#27751
Some users may already be familiar with column stores, so saying more explicitly
that doc values are a columnar representation of the data may help them better
and/or more quickly understand what doc values are about.
Adrien Grand
Yannick Welsch
Clinton Gormley
Rachit Gupta
Ben Tse