Add recommendation for Deployment when HPA is enabled

* Advertise that we need to remove `spec.replicas` when a Horizontal Pod
  Autoscaler is active to prevent unnecessary changes in Pod counts during
  Deployment object changes
* Make note that a Deployment that has this value set behave awkwardly if a
  Deployment is scaled manually outside of the Deployment object

Signed-off-by: John T Skarbek <jtslear@gmail.com>

content/en/docs/concepts/workloads/controllers/statefulset.md

@@ -295,6 +295,22 @@ a Pod is considered ready, see [Container Probes](/docs/concepts/workloads/pods/
 
 Please note that this field only works if you enable the `StatefulSetMinReadySeconds` [feature gate](/docs/reference/command-line-tools-reference/feature-gates/).
 
+### Replicas
+
+`.spec.replicas` is an optional field that specifies the number of desired Pods. It defaults to 1.
+
+Should you manually scale a deployment, example via `kubectl scale
+statefulset statefulset --replicas=X`, and then you update that StatefulSet
+based on a manifest (for example: by running `kubectl apply -f
+statefulset.yaml`), then applying that manifest overwrites the manual scaling
+that you previously did.
+
+If a [HorizontalPodAutoscaler](/docs/tasks/run-application/horizontal-pod-autoscale/)
+(or any similar API for horizontal scaling) is managing scaling for a
+Statefulset, don't set `.spec.replicas`. Instead, allow the Kubernetes
+{{<glossary_tooltip text="control plane" term_id="control-plane" >}} to manage
+the `.spec.replicas` field automatically.
+
 ## {{% heading "whatsnext" %}}