document the migration to enable IPv6 for Docker nodes

[smira]

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This is required for SideroLink. So e.g. upgrading Sidero to 0.5.x might break Sidero installed in Docker.

The fix is https://github.com/siderolabs/talos/blob/19bf12af07aaf6b54d08027676d8a01b4dd4ed29/pkg/provision/providers/docker/node.go#L109-L112

So if possible, patching the docker container with the sysctl stuff should be enough.