ETCDCTL_README: clarify the usage of ETCDCTL_* variables
ETCDCTL_README: clarify the usage of ETCDCTL_* variables

fixes # 10840

mqliang/etcd,gyuho/etcd,cockroachdb/etcd,mqliang/etcd,rnaveiras/etcd,mkumatag/etcd,coreos/etcd,mkumatag/etcd,ajityagaty/etcd,dannysauer/etcd,etcd-io/etcd,gyuho/etcd,rnaveiras/etcd,mqliang/etcd,mkumatag/etcd,rnaveiras/etcd,ajityagaty/etcd,cockroachdb/etcd,etcd-io/etcd,li-ang/etcd,ajityagaty/etcd,etcd-io/etcd,dannysauer/etcd,coreos/etcd,li-ang/etcd