update documentation for fleet field

mmv1/third_party/terraform/website/docs/r/container_cluster.html.markdown

@@ -1282,6 +1282,10 @@ linux_node_config {
 
 * `vulnerability_mode` - (Optional) Sets the mode of the Kubernetes security posture API's workload vulnerability scanning. Available options include `VULNERABILITY_DISABLED`, `VULNERABILITY_BASIC` and `VULNERABILITY_ENTERPRISE`.
 
+<a name="nested_security_posture_config"></a>The `fleet` block supports:
+
+* `project` - (Optional) The name of the Fleet host project where this cluster will be registered.
+
 
 ## Attributes Reference
 
@@ -1326,6 +1330,8 @@ exported:
 
 * `node_config.0.effective_taints` - List of kubernetes taints applied to each node. Structure is [documented above](#nested_taint).
 
+* `fleet.0.membership` - The resource name of the fleet Membership resource associated to this cluster with format `projects/{{project}}/locations/{{location}}/memberships/{{name}}`. See the official doc for [fleet management]((https://cloud.google.com/kubernetes-engine/docs/fleets-overview)). 
+
 ## Timeouts
 
 This resource provides the following