Rewrite help for Kubespawner.cmd

[manics]

Whilst working on jupyterhub/zero-to-jupyterhub-k8s#2138 I noticed the current description for cmd is mostly copied from the parent class and is a bit confusing. I've attempted to simplify it.

[manics]

FYI I picked the original Cmd/Entrypoint casing from https://github.com/opencontainers/image-spec/blob/master/config.md

[consideRatio]

cmd will be passed to the image's ENTRYPOINT.

I think it is a problematic statement for two reasons.

1. It only influence the k8s Pod rather than the Dockerfile or similarly.
2. It actually configures the k8s container's args (like Dockerfile's CMD) rather than command (like Dockerfile's ENTRYPOINT).

A related confusion is described in #493.

I think we can make small progressions and I find this change PR mergable if we just update this to reference that it implies setting the k8s Pod's container's args where the command won't be set. This also implies that any Dockerfile's ENTRYPOINT will always be respected as kubespawner never sets command. Here is related documentation about that.

[consideRatio]

THANK YOU FOR YOUR WORK ON THIS @manics!! ❤️

I forgot to convey that part before!

[consideRatio]

I'll go for a merge on this as it is and work on an entrypoint traitlet PR.