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
This commit adds a `doc_count` field to the response body of Matrix
Stats aggregation. It exposes the number of documents involved in
the computation of statistics, a value that can already be retrieved using
the method MatrixStats.getDocCount() in the Java API.
