[w3c/browser-payment-api] Update "user accepts the payment request algorithm" to include interaction with payment app (#192)

There is an inconsistency in the spec between the description of `PaymentResponse.details` and [user accepts the payment request algorithm](https://w3c.github.io/browser-payment-api/specs/paymentrequest.html#user-accepts-the-payment-request-algorithm)

The description of `PaymentResponse.details` includes the following:
> This data is returned by the payment app that satisfies the payment request. 

However the algorithm is short on detail about:
1. How the user will select a payment app (it only talks about __accepting a payment request__)
1. How the UA will pass the payment request to the payment app
1. How the UA will get the payment response back from the payment app

This is related to issues #50 and #39 but is here to track the spec changes required once these questions have been answered.

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https://github.com/w3c/browser-payment-api/issues/192