[Spec] Write the 'transaction UX' section (#114)

The goal here is to enforce that User Agents do communicate the necessary
information that will be signed over, but not to restrict their implementation
of how to do so.

spec.bs

@@ -514,13 +514,20 @@ input {{SecurePaymentConfirmationRequest}} |request|, are:
 
 ### Displaying a transaction confirmation UX ### {#sctn-transaction-confirmation-ux}
 
-<div class="note">
-**TODO**: We need to determine how best this can be specified. We tend to avoid
-requiring user agents to show specific UX, but in the case of SPC we do want to
-ensure that the appropriate transaction details are communicated to the user
-(either via browser UX or via the authenticator device itself, if it has an
-output).
-</div>
+To avoid restricting User Agent implementation choice, this specification does
+not require a User Agent to display a particular user interface when
+{{PaymentRequest/show|PaymentRequest.show()}} is called and the [=Secure
+Payment Confirmation payment handler=] is selected. However, so that a
+[=Relying Party=] can trust the information included in
+{{CollectedClientPaymentData}}, the User Agent MUST ensure that the following
+is communicated to the user:
+
+* The {{CollectedClientAdditionalPaymentData/payeeOrigin}}.
+* The {{CollectedClientAdditionalPaymentData/total}}, that is the
+    {{PaymentCurrencyAmount/currency}} and {{PaymentCurrencyAmount/value}} of the transaction.
+* The {{CollectedClientAdditionalPaymentData/instrument}} details, that is the
+    payment instrument {{PaymentCredentialInstrument/displayName}} and
+    {{PaymentCredentialInstrument/icon}}.
 
 ### Steps to respond to a payment request ### {#sctn-steps-to-respond-to-a-payment-request}