fix(delete-product-infrastructure): add note on DNS resource records

The Cloud DNS managed zone is a resource shared between the product factory and downstream deployments. As such it is very easy to get into a situation where there is a dangling record, either because the product wasn't fully terraform destroy-ed before the GCP product factory config was removed or because a resource record was manually created.

Google sensibly refuses to remove Cloud DNS managed zones which contain records and it's not entirely clear whose responsibility it is, product factory or per-product configuration, to ensure the zone is empty.

As we hope this is relatively rare, fix the issue through documentation in the first instance.