Add note about resizing to the downward API reference

Author: tallclair

content/en/docs/concepts/workloads/pods/downward-api.md

@@ -99,6 +99,11 @@ These container-level fields allow you to provide information about
 [requests and limits](/docs/concepts/configuration/manage-resources-containers/#requests-and-limits)
 for resources such as CPU and memory.
 
+{{< note >}}
+{{< feature-state feature_gate_name="InPlacePodVerticalScaling" >}}
+Container CPU and memory resources can be resized while the container is running. If this happens, a downward API volume will be updated, but environment variables will not be updated unless the container restarts. See [Resize CPU and Memory Resources assigned to Containers](/docs/tasks/configure-pod-container/resize-container-resources/) for more details.
+{{< /note >}}
+
 
 `resource: limits.cpu`
 : A container's CPU limit