Make clear in the docs that the role mapping APIs is the preferred
way to manage role mappings and that the role mappings that are
defined in files cannot be viewed or managed with the APIs
* This commit is part of our plan to deprecate and ultimately remove the use of _xpack in the REST APIs.
- REST API docs
- HLRC docs and doc tests
- Handle REST actions with deprecation warnings
- Changed endpoints in rest-api-spec and relevant file names
