doc: refine resources doc
## Resource Labels
The etcd-operator creates the following Kubernetes resources for each etcd cluster:
- Pods for the etcd nodes
-- Deployment for the backup side car
-- Services for the etcd client, peer and backup service
-- Persistent Volume Claim, if backup is enabled with the storage type as PersistentVolume
+- Services for the etcd client, peer, and (optional) backup service
+- (Optional) Deployment for the backup sidecar, if backup spec is defined
+- (Optional) Persistent Volume Claim, if backup sidecar is enabled with the storage type as PersistentVolume
where each resource has the following labels:
- `app=etcd`
