bonding: add documentation for peer_notif_delay

mainline inclusion
from mainline-v5.5-rc5
commit 0307d589
category: bugfix
bugzilla: 46933
CVE: NA

-------------------

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>
Signed-off-by: Aichun Li <liaichun@huawei.com>
Reviewed-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: wangxiaopeng <wangxiaopeng7@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>