x/crypto/acme: better document Register behavior when account already exists

[rolandshoemaker]

Currently when Client.Register is called and an account already exists the behavior diverges based on the server implementation. If the server is pre-8555 it just returns the account object and no error, if it is 8555 compliant it returns no account object and a ErrAccountAlreadyExists error. In the latter case it will also store the account KID internally, allowing for follow-up requests (although this is not documented).

Ideally for 8555 servers Client.Register should return the account object when it already exists (perhaps in conjunction with the ErrAccountAlreadyExists error) so that the user doesn't have to do two requests to actually get the object (Client.Regiser and Client.GetReg), but at the least the documentation should be improved to actually say what happens in this circumstance.

[cagedmantis]

/cc @FiloSottile @x1ddos

[gopherbot]

Change https://golang.org/cl/233197 mentions this issue: x/crypto/acme: make Client.Register behavior consistent