Update docs with Concourse hermetic feature

[Rui Yang]

Is your feature request related to a problem? Please describe.

refer to concourse/concourse#8713

Describe the solution you'd like

we should be clear that this feature is meant for containerd runtime only. When setting it with guardian runtime it won't work.

Describe alternatives you've considered

No response

Additional context

Follow what task privileged does.

Also looking for other places that would potentially need a reference to this feature.