adding more cluster API docs.

Change-Id: Idddcc68ad29c1a41c8aeeca6412322deb50e5fb2
Reviewed-on: http://review.couchbase.org/40538
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>