AS far as documentation goes. I'm not sure, certainly it's spare, what the future of this module is and since it's wrapped up quickly by apeisa for simple own project, there isn't much support or development going. Although I find the readme and looking at the example payment modules enough to get it up quickly. I know this doesn't apply to all. Non experienced developers may struggle. But usually I find people scare away from modules (when they shouldn't) and think there's a lot of complexity that isn't there really. It's so simple as it doesn't requires much and works with the same API as much as you would with pages and in templates.
A Payment module here is the module run when you get to the /checkout/payment/ step, it either just saves the order and shows a message or redirects, or it builds a form you need for your payment provider etc .
Look at the simple invoice example.
https://github.com/apeisa/Shop-for-ProcessWire/blob/master/PaymentInvoice.module
processPayment() is the method called once you reach that point in the checkout. This can be anything that is required to do the payment.
https://github.com/apeisa/Shop-for-ProcessWire/blob/master/PaymentExample.module