doc: update switchdev L3 section

This is to reflect the change of FIB offload infrastructure from
switchdev objects to FIB notifier.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>