[webpayments] Confusion over payment flow (#10)

cc: @mattsaxon 

I'm looking over the payment flows, in particular the [3Ds & Google Flow](https://github.com/w3c/webpayments/blob/gh-pages/PaymentFlows/Card/MerchantHosted-CardPaymentwith3DS-GoogleProposal.pml), and I'm not sure I understand the characters/roles that are laid out at the top of the diagram [1]. Namely:

* You seem to be conflating the user and the browser ("Payer (Shopper) Browser"), but these are distinct roles and I'm getting confused who is taking what action. Perhaps you really just mean "Browser" here? Since there's only one browser (or "User-Agent") in any given transaction, I don't think we need to be more specific here.

* I do not know what a "Browser Payment Agent/Wallet" and a "Payer (Shopper) PSP Wallet [aka Issuer Wallet]" are or how they differ.

Given the confusion above, I'm not 100% sure I follow the proposed flow. But it seems to suggest that the user selects a payment instrument from the browser window (I'm using our terminology here, where "payment instrument" is the thing the user selects inside of the browser UI), the user goes into the payment instrument, a blob of data is returned back, which then gets passed back to the merchant, and only then does the user get put through the 3DS verification flow.

While this is certainly one way to do it, I don't think it's preferred or ideal. The user should be put through and 3DS flow inside of the payment instrument *before* any data is returned to the merchant. The goal is to make things easier for merchants and put the complicated logic inside of the payment instrument and whomever is providing that instrument.

Sorry if I've misunderstood anything.

[1] I am using planttext.com to take plain text and convert it into actual diagram, so not 100% sure it's accurate

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https://github.com/w3c/webpayments/issues/10