buildkitd.toml.md should explain the interaction between [worker.oci] and [[worker.oci.gcpolicy]]

[betelgeuse]

https://github.com/moby/buildkit/blob/ab83f87af83d0ff8e5de356812eb530fe49c6e07/docs/buildkitd.toml.md

This file could use a comment that explains the relation for gc settings under [worker.oci] and [[worker.oci.gcpolicy]]. Basically how the various gcpolicies interact with the worker level settings and what ends up being the priority between multiple policies.

  # maxUsedSpace is the maximum amount of disk space that may be used by
  # this buildkit worker - any usage above this threshold will be reclaimed
  # during garbage collection.
  maxUsedSpace = "60%"

    # maxUsedSpace is the maximum amount of disk space that may be used by this
    # policy - any usage above this threshold will be reclaimed during garbage
    # collection.
    maxUsedSpace = "1GB"