Add `immutable` on `PodFailurePolicyRule` in docs

**This is a Bug Report**

**Problem:**

When applying a `PodFailurePolicy` on an already made `CronJob`, it returns a invalid value stating that the `PodFailurePolicy` is an immutable field. This isn't brought up anywhere when checking the docs.

```
Invalid value: batch.PodFailurePolicy{Rules:[]batch.PodFailurePolicyRule(nil)}: field is immutable
```

**Proposed Solution:**
Update docs so that `PodFailurePolicyRule` is an immutable field in CronJobs

**Page to Update:**
https://kubernetes.io/docs/reference/generated/kubernetes-api/v1.29/#podfailurepolicyrule-v1-batch