Better document `ristretto-issuer-root-url`.
Created by: tomprince
From looking at the code, it looks like:
- the client talks to the server at that URL as part of the redemption process, as well as verifies it against the announced URL of the server
- the server just use the configured URL of the server.
Partly I was confused by reading https://github.com/PrivateStorageio/ZKAPAuthorizer/blob/632d2cdc96bb2975d8aff573a3858f1a6aae9963/docs/source/configuration.rst#L35-L36 which suggested that the value was just a label that the client and server needed to agree upon. After having looked at the code and rereading, I see that it also says above that the client talks to an issue server, but I missed that until I had looked at the code.