I did confirm that the documentation is off, and that v1/cart/payments/cart-payment/transactions does return the full Payment model...
Because of this, the OrderCloudClient method Cart.CreatePaymentTransactionAsync() incorrectly (but somehow without error) populates and returns a PaymentTransaction model when it should populate and return a Payment model.
As confirmed by @djsteinmetz in this Sitecore Slack thread:
Because of this, the OrderCloudClient method Cart.CreatePaymentTransactionAsync() incorrectly (but somehow without error) populates and returns a
PaymentTransaction
model when it should populate and return aPayment
model.