This is indeed hard to find, currently. The base is in the “uplink”-repository, where uplink-c is based on.
So for DownloadOptions see here: https://github.com/storj/uplink/blob/7bbef5a093dc73df62e33b6ab80accb34e723c97/download.go#L14
I just yesterday have been in a call with storj and we talked about this one. uplink-c needs a little more documentation here.