add changelog and doc

Author: lyu571

.changelog/2918.txt

@@ -0,0 +1,3 @@
+```release-note:enhancement
+resource/tencentcloud_kubernetes_cluster: support `cluster_os` field to update and adjust its set logic
+```

website/docs/r/kubernetes_cluster.html.markdown

@@ -846,7 +846,7 @@ The following arguments are supported:
 * `cluster_max_service_num` - (Optional, Int, ForceNew) The maximum number of services in the cluster. Default is 256. The range is from 32 to 32768. When its power unequal to 2, it will round upward to the closest power of 2.
 * `cluster_name` - (Optional, String) Name of the cluster.
 * `cluster_os_type` - (Optional, String, ForceNew) Image type of the cluster os, the available values include: 'GENERAL'. Default is 'GENERAL'.
-* `cluster_os` - (Optional, String, ForceNew) Cluster operating system, supports setting public images (the field passes the corresponding image Name) and custom images (the field passes the corresponding image ID). For details, please refer to: https://cloud.tencent.com/document/product/457/68289.
+* `cluster_os` - (Optional, String) Cluster operating system, supports setting public images (the field passes the corresponding image Name) and custom images (the field passes the corresponding image ID). For details, please refer to: https://cloud.tencent.com/document/product/457/68289.
 * `cluster_subnet_id` - (Optional, String, ForceNew) Subnet ID of the cluster, such as: subnet-b3p7d7q5.
 * `cluster_version` - (Optional, String) Version of the cluster. Use `tencentcloud_kubernetes_available_cluster_versions` to get the upgradable cluster version.
 * `container_runtime` - (Optional, String, ForceNew) Runtime type of the cluster, the available values include: 'docker' and 'containerd'.The Kubernetes v1.24 has removed dockershim, so please use containerd in v1.24 or higher.Default is 'docker'.