docs(kubeadm-upgrade): add consideration on etcd upgrade impact

relates to kubernetes/kubeadm#2991 (comment)

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

@@ -54,6 +54,16 @@ The upgrade workflow at high level is the following:
   with the purpose of reconfiguring the cluster is not recommended and can have unexpected results. Follow the steps in
   [Reconfiguring a kubeadm cluster](/docs/tasks/administer-cluster/kubeadm/kubeadm-reconfigure) instead.
 
+### Considerations when upgrading etcd
+
+Because the `kube-apiserver` pod is running at all times (even if you have
+cordoned the node), when you perform a kubeadm upgrade which includes an etcd
+upgrade, in-flight requests will stall while the new etcd pod is restarting.
+As a workaround, it is possible to actively stop (with `kill -s SIGTERM`)
+`kube-apiserver` a few seconds before starting the `kubeadm upgrade apply`
+command: this permits to complete in-flight requests and close existing
+connections, and minimizes the consequence of the `etcd` downtime.
+
 <!-- steps -->
 
 ## Changing the package repository