bonding: update documentation - alternate configuration.

The bonding documentation used to provide configuration
details and examples for initscripts and sysconfig only.

This patch describe the third possible configuration:
/etc/network/interfaces.

Signed-off-by: Nicolas de Pesloüan <nicolas.2p.debian@free.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>