bonding: add documentation for peer_notif_delay

Ability to tweak the interval between peer notifications has been
added in 07a4ddec ("bonding: add an option to specify a delay
between peer notifications") but the documentation was not updated.

Signed-off-by: Vincent Bernat <vincent@bernat.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>