Document the network interactions involved in redeeming a voucher
In https://github.com/PrivateStorageio/PaymentServer/pull/102#discussion_r710387606 @tomprince asked:
Is there documentation anywhere on how a voucher is redeemed a fixed number of times for groups and tokens, and that that is what is meant by "total number of redemption groups"? It took me a while to figure out that that is what is going on here, reading both this code, and the corresponding code in ZKAPAuthorizer.
The answer now is that no, there is no such documentation. Make the answer yes instead.