Document that PVC created by STS does not support auto expansion of volume

[surajssd]

Suppose that you have a Statefulset and a PVC associated with it. Now if you update the storage field in the STS to a new increased number, and if you expect the STS to also update the PVC associated with it then it won't happen. The user has to manually update the PVC size. This is still a WIP in upstream k8s.

• KEP: Add StatefulSet Volume Expansion Kep kubernetes/enhancements#660
• Prometheus Operator issue: https://github.com/prometheus-operator/prometheus-operator/issues/2357

So document this issue with all the storage provider components of Lokomotive and with Prometheus Operator as well, since it deploys prometheus and alertmanager as Statefulset.