* Payment methods describe data; they are not participants in a transaction.
   Therefore, recast the description of parties as payers/payees/providers
   of payment methods. (Both in Intro and Goals)

 * The word "buyer" was used in the intro and not used elsewhere in the
   spec. Therefore, I use payer/payee which are used further down.

 * The first examples the reader should encounter in the PaymentRequest
   interface description should be those most likely found in practice.
   The "Buy" button should come first. The other examples of in-app
   and POS are much further out and so I have listed them after.

 * Updated example 2 as follows:

   - We have network id values; we should use those instead of fictitious
     ones.

   - bobpay.com does not confirm (I believe) to the PMI spec. So I have
     used an example URL instead.

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