socket: Update timestamping Documentation

With the new y2038 safe timestamping options added, update the
documentation to reflect the changes.

Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>