firewall: document rules for IPSec ESP/ISAKMP with 'name' option

These are recommended practices by REC-22 and REC-24 of RFC6092:
"Recommended Simple Security Capabilities in Customer Premises Equipment
(CPE) for Providing Residential IPv6 Internet Service"

Fixes FS#640

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>