[w3c/browser-payment-api] Explicitly state any cases where an iframe with an allowpaymentrequest is not allowed to make payment requests (#358)

The spec currently states that when an `iframe` element has an `allowpaymentrequest` attribute:

> it indicates that scripts in the `iframe` element's browsing context are allowed to make payment requests (if it's not blocked for other reasons, e.g., there is another ancestor `iframe` without this attribute set)

But nowhere does the spec define any “others reasons” it would be “blocked” (from being allowed to making payment requests). I don’t know what the other cases might be, but if there are any other than the cases where the `iframe` has an ancestor `iframe` without an `allowpaymentrequest` attribute, then the spec should explicitly state those cases.

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https://github.com/w3c/browser-payment-api/issues/358