Clarify firewall rule doc.
Clarify firewall rule doc.

coryfklein/ingress,jcmoraisjr/ingress,kubernetes/ingress-gce,nicksardo/ingress,nicksardo/ingress,canhnt/ingress,caiyixiang/ingress,canhnt/ingress,aledbf/ingress-nginx,porridge/ingress,winfield-chen-hs/ingress,caicloud/ingress,literalice/ingress,literalice/ingress,danielqsj/ingress,aledbf/ingress-nginx,kubernetes/ingress-nginx,winfield-chen-hs/ingress,caicloud/ingress,nicksardo/ingress,kubernetes/ingress-nginx,canhnt/ingress,jcmoraisjr/ingress,aledbf/ingress-nginx,nicksardo/ingress,coryfklein/ingress,kubernetes/ingress-nginx,chentao1596/ingress,porridge/ingress,caicloud/ingress,matthewg/ingress,caiyixiang/ingress,caicloud/ingress,canhnt/ingress,arjanschaaf/ingress,nevetS/ingress,matthewg/ingress,arjanschaaf/ingress,kubernetes/ingress-nginx,kubernetes/ingress-nginx,nevetS/ingress,jcmoraisjr/ingress,danielqsj/ingress,kubernetes/ingress-gce,chentao1596/ingress,aledbf/ingress-nginx