Mention type parameter of createReplication REST endpoint.

Change-Id: I89160c71f1680235145f13208119c81f99f76abd
Reviewed-on: http://review.couchbase.org/42145
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>