doc: --skip-flag in the upgrade workflow (#2313)

Co-authored-by: Thomas Tendyck <[email protected]>

docs/docs/workflows/upgrade.md

@@ -63,6 +63,13 @@ You can use the Terraform state backup to restore previous resources in case an
 You can use the Custom Resource (Definition) backup files to restore Custom Resources and Definitions manually (e.g., via `kubectl apply`) if the automatic migration of those resources fails.
 You can use the Helm charts to manually apply upgrades to the Kubernetes resources, should an upgrade fail.
 
+:::note
+
+For advanced users: the upgrade consists of several phases that can be individually skipped through the `--skip-phases` flag.
+The phases are `infrastracture` for the cloud resource management through Terraform, `helm` for the chart management of the microservices, `image` for OS image upgrades, and `k8s` for Kubernetes version upgrades.
+
+:::
+
 ## Check the status
 
 Upgrades are asynchronous operations.