user-namespaces.md: subid count per pod is hard-coded to 65536

The number of subuids and subgids for each of pods is hard-coded to 65536,
regardless to the total ID count specified in `/etc/subuid` and `/etc/subgid`:
https://github.com/kubernetes/kubernetes/blob/v1.32.0/pkg/kubelet/userns/userns_manager.go#L211-L228

This fact was not clarified in the documentation.

Co-authored-by: Tim Bannister <[email protected]>
Signed-off-by: Akihiro Suda <[email protected]>

Author: AkihiroSuda

content/en/docs/concepts/workloads/pods/user-namespaces.md

@@ -177,6 +177,8 @@ to the `kubelet` user:
   configuration.
 
 * The subordinate ID count must be a multiple of 65536
+  (for Kubernetes {{< skew currentVersion >}} the subordinate ID count for each Pod is hard-coded
+  to 65536).
 
 * The subordinate ID count must be at least `65536 x <maxPods>` where `<maxPods>`
   is the maximum number of pods that can run on the node.