docs/libvirt: Add more hints about cleanup and reinstallation

The stale `auth` directory was what was breaking my cluster reinstalls.
Let's encourage people to use `--dir`.

docs/dev/libvirt-howto.md

@@ -235,6 +235,12 @@ openshift-install destroy cluster
 
 If you did not compile with `libvirt_destroy`, you can use [`virsh-cleanup.sh`](../../scripts/maintenance/virsh-cleanup.sh), but note it will currently destroy *all* libvirt resources.
 
+Note that you almost certainly also want to clean up the installer
+state files too, including `auth/`, `terraform.tfstate` etc.  The
+best thing to do is always pass the `--dir` argument to `install`
+and `destroy`.  And if you want to reinstall from scratch, `rm -rf`
+the state directory beforehand.
+
 ### Firewall
 
 With the cluster removed, you no longer need to allow libvirt nodes to reach your `libvirtd`. Restart