Document how ArgC authorization functions work

[aybehrouz]

ArgC authorization functions are the main tool for authorizing operations in the Argennon platform. They can be used for authorizing operations by accounts or applications. Their current implementation can be found here. We need to exactly describe how the end user can use them and how their internal data including public keys and nonce data are stored and updated. In other words, the internal data structure needs to be exactly described.