Add docs on wildcard registry

[brandond]

Technically this has been supported by containerd for a long time, but we never documented or tested it. Ref:

• https://github.com/containerd/containerd/blob/release/1.7/docs/hosts.md#setup-default-mirror-for-all-registries

  Optionally the _default registry host namespace can be used as a fallback, if no other namespace matches.

• https://github.com/containerd/containerd/blob/release/1.6/docs/cri/registry.md#configure-registry-endpoint

  * is configured, and gcr.io is not: endpoints for * + default endpoint https://gcr.io/v2.

Also adds a mention of the two unsupported K3S_PTP env vars.

[brandond]

Spell check failed on release notes, not from this PR.