Update reference docs for docker stack deploy
Update reference docs for docker stack deploy

`docker stack deploy` now support multiple composefil. This updates the
reference doc to take that fact into account.

Signed-off-by: Vincent Demeester <87e1f221a672a14a323e57bb65eaea19d3ed3804@sbr.pm>

docker/cli,dnephin/cli,thaJeztah/cli,dnephin/cli,cpuguy83/cli,docker/cli,cpuguy83/cli,thaJeztah/cli