Comment 1 for bug 2028917

We are currently misleading people by saying that:

```
Once all regions in cluster are configured, use the following command to migrate secrets:

sudo maas config-vault migrate
```

when the user has reconfigured MAAS to point at a new Vault.

Make it much clearer in the error message and confirmation step that

a) the secrets in <OLD-VAULT> will no longer be accessible by MAAS
b) it's the user's responsibility to move secrets from <OLD-VAULT> to <NEW-VAULT> and they should refer to Vault documentation for doing so.
c) retain the message when we are migrating secrets from MAAS to Vault

Docs should be clarified that `config-vault migrate` is *just* about the one way migration from MAAS to Vault and shouldn't be needed to be run more than once.