doc(k8s): private_network_id (#1987)

* doc(k8s): private_network_id 

changing this field won't automatically recreate cluster
linked commit 4ac9d10

* docs: remove trailing space

---------

Co-authored-by: Nathanael Demacon <[email protected]>

Author: louisportay

docs/resources/k8s_cluster.md

@@ -280,8 +280,8 @@ If you prefer keeping it, you should instead set it as `false`.
 
 - `private_network_id` - (Optional) The ID of the private network of the cluster.
 
-~> **Important:** This field can only be set at cluster creation and cannot be updated later.
-Changes to this field will cause the cluster to be destroyed then recreated.
+~> **Important:** This field can be set at cluster creation or later to migrate to a Private Network.
+Any subsequent change after this field got set will prompt for cluster recreation.
 
 - `default_pool` - (Deprecated) See below.