Documentation/op-guide: clarify security.md on TLS auth
Documentation/op-guide: clarify security.md on TLS auth

Make it more accurate (just as pkg/transport/listener_tls.go does).

Signed-off-by: Gyuho Lee <3454051ee3341eb69bdf6fca40899cdad4d86496@gmail.com>

fanminshi/etcd,mkumatag/etcd,mkumatag/etcd,mqliang/etcd,gyuho/etcd,glevand/coreos--etcd,coreos/etcd,whuwxl/etcd,rnaveiras/etcd,heyitsanthony/etcd,ajityagaty/etcd,rnaveiras/etcd,dannysauer/etcd,ajityagaty/etcd,dannysauer/etcd,mqliang/etcd,heyitsanthony/etcd,glevand/coreos--etcd,joshix/etcd,fanminshi/etcd,li-ang/etcd,gyuho/etcd,sebrandon1/etcd,xiang90/etcd,coreos/etcd,vimalk78/etcd,sinsharat/etcd,xiang90/etcd,lorneli/etcd,mqliang/etcd,etcd-io/etcd,lorneli/etcd,bdarnell/etcd,etcd-io/etcd,cockroachdb/etcd,cockroachdb/etcd,whuwxl/etcd,ajityagaty/etcd,rnaveiras/etcd,bdarnell/etcd,vimalk78/etcd,sinsharat/etcd,joshix/etcd,li-ang/etcd,sebrandon1/etcd,mkumatag/etcd,etcd-io/etcd