kubeadm/init-join.md: add note about using "skipPhases" from config

Since 1.22 it will be possible to skip phases using the configuration
file. Add note about that in the relevant sections of the kubeadm
init and join reference pages.

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

@@ -104,6 +104,10 @@ sudo kubeadm init --skip-phases=control-plane,etcd --config=configfile.yaml
 
 What this example would do is write the manifest files for the control plane and etcd in `/etc/kubernetes/manifests` based on the configuration in `configfile.yaml`. This allows you to modify the files and then skip these phases using `--skip-phases`. By calling the last command you will create a control plane node with the custom manifest files.
 
+{{< feature-state for_k8s_version="v1.22" state="beta" >}}
+
+Alternatively, you can use the `skipPhases` field under `InitConfiguration`.
+
 ### Using kubeadm init with a configuration file {#config-file}
 
 {{< caution >}}

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

@@ -66,6 +66,10 @@ For example:
 sudo kubeadm join --skip-phases=preflight --config=config.yaml
 ```
 
+{{< feature-state for_k8s_version="v1.22" state="beta" >}}
+
+Alternatively, you can use the `skipPhases` field in `JoinConfiguration`.
+
 ### Discovering what cluster CA to trust
 
 The kubeadm discovery has several options, each with security tradeoffs.