Document new 2.1 API [JIRA: PMT-179]

[seemaj]

• riak_core_ring_manager:is_stable_ring/0
  Add riak_core_ring_manager:is_stable_ring/0 public API [JIRA: RIAK-1598] riak_core#716
• API to retrieve active preflist based on particular bucket/key
  Updates the Riak API with an operation to GET, e.g. /types/Type/buckets/Bucket/keys/Key/preflist, an annotated (nodes being primary or fallback) preflist of which nodes host a particular bucket/key combination is returned.
  • riak-erlang-http-client/pull/50
  • riak_core/pull/705
  • riak_pb/pull/105
  • riak-erlang-client/pull/204
  • riak_kv/pull/1083
  • riak_api/pull/81
  • riak_api/pull/75