Add documentation for the Helm Operator OOM issue (#3470)

Add documentation to advise users to watch single namespace or increase memory limit in case of an `OOMkill` event

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

docs/content/installation/installation-with-operator.md

@@ -61,3 +61,5 @@ $ kubectl apply -f nginx-ingress-controller.yaml
 A new instance of the NGINX Ingress Controller will be deployed by the NGINX Ingress Operator in the `default` namespace with default parameters.
 
 To configure other parameters of the NginxIngressController resource, check the [documentation](https://github.com/nginxinc/nginx-ingress-helm-operator/blob/v1.2.1/docs/nginx-ingress-controller.md).
+
+**Note:** Some users reported an `OOMkilled` error when they deployed the NGINX Ingress Operator in a large cluster with multiple namespaces and Kubernetes objects. This is due to the helm operator caching every Kubernetes object in the cluster, and thus consuming too much system memory. If you encounter this issue, consider setting the operator to only watch one namespace. If watching multiple namespaces is required in your use case, try manually increasing the memory limit for the operator. Note that the value might be overwritten after a release update. We are working with the OpenShift team to resolve this issue.