Ensure all config has help strings for our configuration reference docs

[consideRatio]

The following is traitlet based config without a help string, we should have a help string for all available configuration I think. Currently they emit warnings when we build docs.

ClusterConfig.environment
ClusterConfig.scheduler_cmd
KubeClusterConfig.worker_extra_pod_config
KubeClusterConfig.worker_extra_pod_labels
KubeBackend.common_annotations
KubeBackend.common_labels
KubeController.proxy_web_middlewares

[consideRatio]

This is a bug related to sphinx and autodoc_trait and such, there was actually help strings for these config options.

This is tracked also in jupyterhub/kubespawner#788