Missing documentation for `--config*` flags for `docker service ...`

[jlhawn]

The current repo doesn't document how to use the --config flag when running docker service create ... nor does it document how to use either the --config-rm or --config-add flags when running docker service update ....

Links:

• Documentation for docker service create
• Documentation for docker service update

[thaJeztah]

The flags were added to the reference docs in #403, and this section of the documentation handles using configs; https://docs.docker.com/engine/swarm/configs/

Closing this one, but some additional information in the command line reference docs could still be needed