[w3c/browser-payment-api] Spec prohibits currency validation but doesn't define what it is (#175)

In the definition of [`CurrencyAmount`](https://w3c.github.io/browser-payment-api/specs/paymentrequest.html#currencyamount), the spec defines the [`currency`](https://w3c.github.io/browser-payment-api/specs/paymentrequest.html#dfn-currency) field saying:
> user agents MUST not attempt to validate this string.

The spec does not describe what it means to validate the string. It would be better if instead of a negative, the spec could describe positively what MUST happen. Presumably there is something bad that might happen as a result of some kind of validation? The spec is trying to avoid this and make sure that something does happen but it doesn't say what this is.

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