Docs/correct discover kubernetes constants example (#5900)

* Update example limiting pods on the same node

* Update example syntax

* Add note about helm chart

* Minor syntax tidy

(cherry picked from commit 0ecc190)

docs/sources/flow/reference/components/discovery.kubernetes.md

@@ -462,14 +462,20 @@ Replace the following:
 
 ### Limit to only pods on the same node
 
-This example limits the search to pods on the same node as this {{< param "PRODUCT_ROOT_NAME" >}}. This configuration could be useful if you are running {{< param "PRODUCT_ROOT_NAME" >}} as a DaemonSet:
+This example limits the search to pods on the same node as this {{< param "PRODUCT_ROOT_NAME" >}}.
+This configuration could be useful if you are running {{< param "PRODUCT_ROOT_NAME" >}} as a DaemonSet.
+
+{{% admonition type="note" %}}
+This example assumes you have used Helm chart to deploy {{< param "PRODUCT_NAME" >}} in Kubernetes and sets `HOSTNAME` to the Kubernetes host name.
+If you have a custom Kubernetes deployment, you must adapt this example to your configuration.
+{{% /admonition %}}
 
 ```river
 discovery.kubernetes "k8s_pods" {
   role = "pod"
   selectors {
     role = "pod"
-    field = "spec.nodeName=" + constants.hostname
+    field = "spec.nodeName=" + coalesce(env("HOSTNAME"), constants.hostname)
   }
 }
 
@@ -489,6 +495,7 @@ prometheus.remote_write "demo" {
   }
 }
 ```
+
 Replace the following:
   - `PROMETHEUS_REMOTE_WRITE_URL`: The URL of the Prometheus remote_write-compatible server to send metrics to.
   - `USERNAME`: The username to use for authentication to the remote_write API.