Update Istio DNS certificate document for Istio 1.5 (#6871)
Update Istio DNS certificate document for Istio 1.5 (#6871)

* Update Istio DNS certificate document for Istio 1.5

* Revise overview based on review comments
istio/istio.io,istio/istio.io,istio/istio.io,istio/istio.io,istio/istio.io,istio/istio.io,istio/istio.io