Document Kubernetes support for `docker stack` commands.

[mdlinville]

See docker/cli#721

Closing this ticket due to its age, and the impending refactor. If you think this is in error, feel free to reopen. Thanks!

[docker-robott]

Closed issues are locked after 30 days of inactivity.
This helps our team focus on active issues.

If you have found a problem that seems similar to this, please open a new issue.

/lifecycle locked