docs(upgrading-deis): graceful upgrade in stateless mode
docs(upgrading-deis): graceful upgrade in stateless mode

Graceful upgrade in stateless mode has been fixed in #4506 and the option `--stateless` has been added to `upgrade-prep` and `upgrade-takeover` subcommands.
Update documentation for warning user to use this option.

deis/workflow,deis/workflow,deis/workflow