CBQE-0: adding documentation for new xdcr base tests methods

Change-Id: Ibb2403b4b9affff4784e5dd6c98e843337914d17
Reviewed-on: http://review.couchbase.org/43404
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Aruna Piravi <aruna@couchbase.com>