kubeadm: add docs for UpgradeAddonsBeforeControlPlane feature gate

Author: SataQiu

content/en/docs/reference/setup-tools/kubeadm/kubeadm-init.md

@@ -189,6 +189,30 @@ or `kubeadm upgrade apply`), kubeadm respects the value of `UnversionedKubeletCo
 (during `kubeadm join`, `kubeadm reset`, `kubeadm upgrade ...`), kubeadm attempts to use unversioned ConfigMap name first;
 if that does not succeed, kubeadm falls back to using the legacy (versioned) name for that ConfigMap.
 
+List of deprecated feature gates:
+
+{{< table caption="kubeadm deprecated feature gates" >}}
+Feature | Default 
+:-------|:--------
+`UpgradeAddonsBeforeControlPlane` | `false`
+{{< /table >}}
+
+Feature gate descriptions:
+
+`UpgradeAddonsBeforeControlPlane`
+: This is as a **disabled** feature gate that was introduced for Kubernetes v1.28, in order to allow reactivating a legacy
+and deprecated behavior during cluster upgrade. For kubeadm versions prior to v1.28, kubeadm upgrades cluster addons (including
+CoreDNS and kube-proxy) immediately during `kubeadm upgrade apply`, regardless of whether there are other control plane
+instances that have not been upgraded. This may cause compatibility problems. Since v1.28, kubeadm defaults to a mode that
+always checks whether all the control plane instances have been upgraded before starting to upgrade the addons. This behavior
+is applied to both `kubeadm upgrade apply` and `kubeadm upgrade node`. kubeadm determines whether a control plane instance
+has been upgraded by checking whether the image of the kube-apiserver Pod has been upgraded. You must perform control plane
+instances upgrade sequentially or at least ensure that the last control plane instance upgrade is not started until all the
+other control plane instances have been upgraded completely, and the addons upgrade will be performed after the last control plane
+instance is upgraded. The deprecated `UpgradeAddonsBeforeControlPlane` feature gate gives you a chance to keep the old upgrade
+behavior. You should not need this old behavior; if you do, you should consider changing your cluster or upgrade processes, as this
+feature gate will be removed in a future release.
+
 ### Adding kube-proxy parameters {#kube-proxy}
 
 For information about kube-proxy parameters in the kubeadm configuration see:

content/en/docs/tasks/administer-cluster/kubeadm/kubeadm-upgrade.md

@@ -152,6 +152,20 @@ Pick a control plane node that you wish to upgrade first. It must have the `/etc
    [upgrade/kubelet] Now that your control plane is upgraded, please proceed with upgrading your kubelets if you haven't already done so.
    ```
 
+   {{< note >}}
+   For versions earlier than v1.28, kubeadm defaulted to a mode that upgrades the addons (including CoreDNS and kube-proxy)
+   immediately during `kubeadm upgrade apply`, regardless of whether there are other control plane instances that have not
+   been upgraded. This may cause compatibility problems. Since v1.28, kubeadm defaults to a mode that checks whether all
+   the control plane instances have been upgraded before starting to upgrade the addons. You must perform control plane
+   instances upgrade sequentially or at least ensure that the last control plane instance upgrade is not started until all
+   the other control plane instances have been upgraded completely, and the addons upgrade will be performed after the last
+   control plane instance is upgraded. If you want to keep the old upgrade behavior, please enable the `UpgradeAddonsBeforeControlPlane`
+   feature gate by `kubeadm upgrade apply --feature-gates=UpgradeAddonsBeforeControlPlane=true`. The Kubernetes project does
+   not in general recommend enabling this feature gate, you should instead change your upgrade process or cluster addons so
+   that you do not need to enable the legacy behavior. The `UpgradeAddonsBeforeControlPlane` feature gate will be removed in
+   a future release.
+   {{</ note >}}
+
 1. Manually upgrade your CNI provider plugin.
 
    Your Container Network Interface (CNI) provider may have its own upgrade instructions to follow.