clientv3: Clarify lessor KeepAlive docs
clientv3: Clarify lessor KeepAlive docs

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