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-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.