etcdserver: clarify read index warnings
etcdserver: clarify read index warnings

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

etcd-io/etcd,ajityagaty/etcd,mqliang/etcd,li-ang/etcd,rnaveiras/etcd,rnaveiras/etcd,joshix/etcd,ajityagaty/etcd,cockroachdb/etcd,vimalk78/etcd,mkumatag/etcd,coreos/etcd,cockroachdb/etcd,mqliang/etcd,joshix/etcd,xiang90/etcd,mkumatag/etcd,etcd-io/etcd,xiang90/etcd,sinsharat/etcd,rnaveiras/etcd,gyuho/etcd,dannysauer/etcd,gyuho/etcd,vimalk78/etcd,mqliang/etcd,sinsharat/etcd,ajityagaty/etcd,etcd-io/etcd,mkumatag/etcd,li-ang/etcd,coreos/etcd,dannysauer/etcd