Address the issue raised in
https://github.com/kubernetes/kubernetes/issues/73808, where it became
clear it would be helpful for there to be a reminder for the user that
they are responsible for managing the StatefulSet which provides DNS
names for the pods in a StatefulSet.
StatefulSet deletion does not have any guarantees of orderly termination of pods as is suggested by the documentation. Prior to 1.11 kubectl did scale down StatefulSets before deleting them, but this went away after the removal of reapers. Ref discussion in #68627.
This change updates the documentation to reflect this.