Followup from MR984 on doPayment - further clarifications

This follows up from !984 (merged)

@mattwire ignore my review of yours as that's merged now, but here's my take on a further set of improvements. I've tried to reduce the code, and especially weird specifics that I consider unhelpful - all that qfKey stuff which I want to see the back of as it assumes there's only one process that should need to use a payment processor and it's name is QuickForms.