Add documentation for audit webhook variables (#7434)

* Add documentation for audit webhook variables

* Enclose the value of  audit_webhook_server_url in a codeblock

* Add default value for audit_webhook_batch_max_wait

docs/vars.md

@@ -164,6 +164,14 @@ node_taints:
   * `audit_policy_file`: "{{ kube_config_dir }}/audit-policy/apiserver-audit-policy.yaml"
 
   By default, the `audit_policy_file` contains [default rules](https://github.com/kubernetes-sigs/kubespray/blob/master/roles/kubernetes/control-plane/templates/apiserver-audit-policy.yaml.j2) that can be overridden with the `audit_policy_custom_rules` variable.
+* *kubernetes_audit_webhook* - When set to `true`, enables the webhook audit backend.
+  The webhook parameters can be tuned via the following variables (which default values are shown below):
+  * `audit_webhook_config_file`: "{{ kube_config_dir }}/audit-policy/apiserver-audit-webhook-config.yaml"
+  * `audit_webhook_server_url`: `"https://audit.app"`
+  * `audit_webhook_server_extra_args`: {}
+  * `audit_webhook_mode`: batch
+  * `audit_webhook_batch_max_size`: 100
+  * `audit_webhook_batch_max_wait`: 1s
 
 ### Custom flags for Kube Components