clientv3/doc: Update error handling godoc
updated error handling for
`ErrEmptyKey` but missed the godoc.
// // ctx is canceled by another routine
// } else if err == context.DeadlineExceeded {
// // ctx is attached with a deadline and it exceeded
+// } else if err == rpctypes.ErrEmptyKey {
+// // client-side error: key argument is empty
// } else if ev, ok := status.FromError(err); ok {
// code := ev.Code()
// if code == codes.DeadlineExceeded {
// // server-side context might have timed-out first (due to clock skew)
// // while original client-side context is not timed-out yet
// }
-// } else if verr, ok := err.(*v3rpc.ErrEmptyKey); ok {
-// // process (verr.Errors)
// } else {
// // bad cluster endpoints, which are not etcd servers
// }
