CORE-4876: Implemented documentation for a new endpoint

This request contains the documentation for a new endpoint. 
This endpoint will essentially act as a front-end to RIAK, and will replace some existing endpoints.
I removed the documentation for the _session_ and _search history_ endpoints. 

The _session_ endpoint will be replaced with the new _user-data_ endpoint and will have a reserved key. 

The _search history_ endpoint will no longer be necessary once CORE-4876 is complete.

I also took a different tact when documenting this endpoint. I created a header for the endpoint, and sub headers for the supported HTTP methods. It is my intent that this documentation be sufficient enough for someone to understand how to implement it.


Please review the documentation in this change.