Document JUPYTER_PREFER_ENV_PATH=0 for shared user environments

[minrk]

closes #4243

The default behavior may change, making this less likely to come up by default, but it's still a valid topic and option to set. I don't think there's any more in-scope for jupyterhub for #4243 than to document the option and when it would be useful.

related:

• Feature request: do not prefer env path if it is not writeable jupyter/jupyter_core#318
• Only prefer envs owned by the current user jupyter/jupyter_core#323
• Attempts to write workspaces to non-writable workspaces dir in shared env jupyterlab/jupyterlab#13580
• use jupyter_config_dir instead of config_path[0] for workspaces, settings jupyterlab/jupyterlab#13589

[manics]

I have to admit I still find jupyter configuration more confusing than it should be 😄