nixos/doc: clarify scope of debugHook feature in the test driver (#426935)

## Accessing VMs in the sandbox with SSH {#sec-test-sandbox-breakpoint}

::: {.note}

For debugging with SSH access into the machines, it's recommended to try using

[the interactive driver](#sec-running-nixos-tests-interactively) with its

[SSH backdoor](#sec-nixos-test-ssh-access) first.

This feature is mostly intended to debug flaky test failures that aren't

reproducible elsewhere.

:::

As explained in [](#sec-nixos-test-ssh-access), it's possible to configure an

SSH backdoor based on AF_VSOCK. This can be used to SSH into a VM of a running

build in a sandbox.