core: rephrase condition for Transaction Confirmation
VASPs can potentially shoot themselves in the foot by using legacy (pre-SegWit) transactions or applying RBF after communicating the txid to the counterparty VASP.
Therefore, we should recommend a workflow with less pitfalls. It is still at a VASP's discretion to send the Transaction Confirmation earlier as it is in their full control to use SegWit and avoid RBF.
Closes #52 (closed)