docs(vulnerability-policy): add admission control stage documentation

Author: tembleking

website/docs/r/secure_vulnerability_policy.md

@@ -26,6 +26,15 @@ resource "sysdig_secure_vulnerability_policy" "vulnerability_policy_example" {
       scope = "container.image != ''"
     }
   }
+
+  stages {
+    name = "admission_control"
+    configuration {
+      scope                = "kubernetes.cluster.name = 'my-cluster'"
+      failure_action       = "reject"
+      unknown_image_action = "rejectAndScan"
+    }
+  }
 }
 ```
 
@@ -38,12 +47,14 @@ resource "sysdig_secure_vulnerability_policy" "vulnerability_policy_example" {
 
 ### Stages block
 
-* `name` - (Required) Must be one of `pipeline`, `registry`, or `runtime`.
+* `name` - (Required) Must be one of `pipeline`, `registry`, `runtime`, or `admission_control`.
 * `configuration` - (Optional) Configuration block for the stage. If no configuration is provided, it will apply to any workload in this stage.
 
 ### Configuration block
 
 * `scope` - (Required) Scope expression defining the stage applicability.
+* `failure_action` - (Optional) Required for `admission_control` stage only. Policy Failure Action. What should happen if the policy fails (aka: there's a rule vioation). Must be one of `reject` or `warn`.
+* `unknown_image_action` - (Optional) Required for `admission_control` stage only. Unknown Image Action. What should happen if the image is unknown. Must be one of `reject`, `rejectAndScan`, or `warn`.
 
 ## Attributes Reference