[w3c/payment-handler] interaction with payment methods doesn't appear to be defined (#249)

The [definition of `PaymentInstrument`](https://w3c.github.io/payment-handler/#paymentinstrument-dictionary) defines a `capabilities` member that is described as:

> The capabilities member is a list of payment-method-specific capabilities that this payment handler is capable of supporting for this instrument. For example, for the basic-card payment method, this object will consist of an object with two fields: one for supportedNetworks, and another for supportedTypes.

This seems to define how it works by example.  I would expect something to say which specification is responsible for the definition for each payment method.  For example, it's common for a specification like this to define a term that each specification for a payment method would then have to reference in order to define how it interacts here.

It's also not clear how `capabilities` interactions with the possibility that `enabledMethods` has more than one member.

(I got here from w3ctag/design-reviews#231.)

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