Documentation: bonding: fix the doc of peer_notif_delay

Bonding only supports setting peer_notif_delay with miimon set.

Fixes: 0307d589 ("bonding: add documentation for peer_notif_delay")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>