distribution(managed): add more detail on preparing customer account
Created by: bobheadxi
Messed up this process a little (https://github.com/sourcegraph/customer/issues/410) - email don't get sent because we don't have SMTP on managed instances set up by default (doh!). Also I originally took these handover steps to mean handing over the sourcegraph-admin
account, which seems to not be the case.