[w3c/browser-payment-api] Unclear what happens when multiple PRs are created (#462)

Unless I missed it, it's unclear in the spec what happens when:

```
const pr1 = new PaymentRequest(methods, details, options);
const pr2 = new PaymentRequest(methods, details, options);
p1.show();
p2.show();
```

Should p2 reject? or should it wait till p1 finishes? 

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