tor-chanmgr: change `RequestCancelled` retry from `Never` to `Immediate`

If the channel request was cancelled, I don't see why we shouldn't be able to retry immediately. Additionally, the kind() of RequestCancelled is TransientFailure, and I don't think it make sense for a transient error to return a retry_time() of Never.

Also added a comment to ErrorKind::TransientFailure.

(A RequestCancelled error is created at: https://gitlab.torproject.org/tpo/core/arti/-/blob/d8aa4fc4c85ebd95f2331ffeead9380e21bdc75c/crates/tor-chanmgr/src/mgr.rs#L247)