bonding: document two undocumented options.

Commit 655f8919
    bonding: add min links parameter to 802.3ad

and commit ebd8e497
    bonding: add all_slaves_active parameter

introduced new options to bonding, but didn't provide the documentation
for those options.

V2: add the default value for both options.
V3: document the exact behavior of min_links default value.

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