missing runner types on https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/blob/master/docs/executors/README.md#imnotsure

gitlab-ci-multi-runner register suggests me:

Please enter the executor: shell, ssh, virtualbox, docker+machine, docker-ssh+machine, docker, docker-ssh, parallels:

but the documentation only offers

Shell 	Docker 	Docker-SSH 	VirtualBox 	Parallels 	SSH 	Kubernetes

what is docker+machine supposed to be?