[Architecture] Updated wiki to address some comments

Hi all,

I have updated the following wiki page to address some comments:

https://github.com/w3c/webpayments/wiki/A-Payments-Initiation-Architecture-for-the-Web

The key message in this change is that there is not supposed to be a 1:1
comparison between payment app and payment instrument. Rather the payment
mediator and payments apps replace the idea of a digital wallet because of
the shortcomings of the wallet metaphor.

Please have a look at the new page and provide feedback via the existing
issues (or new ones).

Adrian