modifying docs for SidecarContainers beta graduation (#43471)

Signed-off-by: Matthias Bertschy <matthias.bertschy@gmail.com>

content/en/docs/concepts/workloads/pods/_index.md

@@ -111,9 +111,9 @@ Some Pods have {{< glossary_tooltip text="init containers" term_id="init-contain
 as well as {{< glossary_tooltip text="app containers" term_id="app-container" >}}.
 By default, init containers run and complete before the app containers are started.
 
-{{< feature-state for_k8s_version="v1.28" state="alpha" >}}
+{{< feature-state for_k8s_version="v1.29" state="beta" >}}
 
-Enabling the `SidecarContainers` [feature gate](/docs/reference/command-line-tools-reference/feature-gates/)
+Enabled by default, the `SidecarContainers` [feature gate](/docs/reference/command-line-tools-reference/feature-gates/)
 allows you to specify `restartPolicy: Always` for init containers.
 Setting the `Always` restart policy ensures that the init containers where you set it are
 kept running during the entire lifetime of the Pod.

content/en/docs/concepts/workloads/pods/init-containers.md

@@ -291,9 +291,9 @@ validation error is thrown for any container sharing a name with another.
 
 #### API for sidecar containers
 
-{{< feature-state for_k8s_version="v1.28" state="alpha" >}}
+{{< feature-state for_k8s_version="v1.29" state="beta" >}}
 
-Starting with Kubernetes 1.28 in alpha, a feature gate named `SidecarContainers`
+Enabled by default with Kubernetes 1.29, a feature gate named `SidecarContainers`
 allows you to specify a `restartPolicy` for init containers which is independent of
 the Pod and other init containers. Container [probes](/docs/concepts/workloads/pods/pod-lifecycle/#types-of-probe)
 can also be added to control their lifecycle.
@@ -376,4 +376,4 @@ Kubernetes, consult the documentation for the version you are using.
 * Read about [creating a Pod that has an init container](/docs/tasks/configure-pod-container/configure-pod-initialization/#create-a-pod-that-has-an-init-container)
 * Learn how to [debug init containers](/docs/tasks/debug/debug-application/debug-init-containers/)
 * Read about an overview of [kubelet](/docs/reference/command-line-tools-reference/kubelet/) and [kubectl](/docs/reference/kubectl/)
-* Learn about the [types of probes](/docs/concepts/workloads/pods/pod-lifecycle/#types-of-probe): liveness, readiness, startup probe.
+* Learn about the [types of probes](/docs/concepts/workloads/pods/pod-lifecycle/#types-of-probe): liveness, readiness, startup probe.

content/en/docs/concepts/workloads/pods/pod-lifecycle.md

@@ -504,6 +504,22 @@ termination grace period _begins_. The behavior above is described when the
 feature gate `EndpointSliceTerminatingCondition` is enabled.
 {{</note>}}
 
+{{<note>}}
+Beginning with Kubernetes 1.29, if your Pod includes one or more sidecar containers
+(init containers with an Always restart policy), the kubelet will delay sending
+the TERM signal to these sidecar containers until the last main container has fully terminated.
+The sidecar containers will be terminated in the reverse order they are defined in the Pod spec.
+This ensures that sidecar containers continue serving the other containers in the Pod until they are no longer needed.
+
+Note that slow termination of a main container will also delay the termination of the sidecar containers.
+If the grace period expires before the termination process is complete, the Pod may enter emergency termination.
+In this case, all remaining containers in the Pod will be terminated simultaneously with a short grace period.
+
+Similarly, if the Pod has a preStop hook that exceeds the termination grace period, emergency termination may occur.
+In general, if you have used preStop hooks to control the termination order without sidecar containers, you can now
+remove them and allow the kubelet to manage sidecar termination automatically.
+{{</note>}}
+
 1. When the grace period expires, the kubelet triggers forcible shutdown. The container runtime sends
    `SIGKILL` to any processes still running in any container in the Pod.
    The kubelet also cleans up a hidden `pause` container if that container runtime uses one.
@@ -584,4 +600,4 @@ for more details.
 
 * For detailed information about Pod and container status in the API, see
   the API reference documentation covering
-  [`status`](/docs/reference/kubernetes-api/workload-resources/pod-v1/#PodStatus) for Pod.
+  [`status`](/docs/reference/kubernetes-api/workload-resources/pod-v1/#PodStatus) for Pod.

content/en/docs/reference/command-line-tools-reference/feature-gates.md

@@ -185,7 +185,8 @@ For a reference to old feature gates that are removed, please refer to
 | `SELinuxMountReadWriteOncePod` | `true` | Beta | 1.28 | |
 | `SchedulerQueueingHints` | `true` | Beta | 1.28 | |
 | `SecurityContextDeny` | `false` | Alpha | 1.27 | |
-| `SidecarContainers` | `false` | Alpha | 1.28 | |
+| `SidecarContainers` | `false` | Alpha | 1.28 | 1.28 |
+| `SidecarContainers` | `true` | Beta | 1.29 | |
 | `SizeMemoryBackedVolumes` | `false` | Alpha | 1.20 | 1.21 |
 | `SizeMemoryBackedVolumes` | `true` | Beta | 1.22 | |
 | `StableLoadBalancerNodeSet` | `true` | Beta | 1.27 | |