bonding: documentation update: mailing lists.

In commit a6c36ee6 ("bonding: change list
contact to netdev@vger.kernel.org"), the mailing list for bonding
developpement was changed from bonding-devel to netdev.

Update the bonding documentation to reflect this change:

- bonding-devel is used for usage discussions (despite the name).
- netdev is used for developpement discussions.

Also remove the reference to the sourceforge bonding page, which is
deprecated.

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