ImagePolicyWebhook: config can be embedded (#15233)

All admission control plugins support two modes for specifying their
configuration: linking to an external file using the `path` key in the
shared admission configuration file, or directly embedding the
configuration using the `configuration` key in the shared admission
configuration file.

This commit makes the ImagePolicyWebhook documentation mention the
embedded configuration option.

content/en/docs/reference/access-authn-authz/admission-controllers.md

@@ -249,6 +249,22 @@ plugins:
 ...
 ```
 
+Alternatively, you can embed the configuration directly in the file:
+
+```yaml
+apiVersion: apiserver.k8s.io/v1alpha1
+kind: AdmissionConfiguration
+plugins:
+- name: ImagePolicyWebhook
+  configuration:
+    imagePolicy:
+      kubeConfigFile: /path/to/file
+      allowTTL: 50
+      denyTTL: 50
+      retryBackoff: 500
+      defaultAllow: true
+```
+
 The ImagePolicyWebhook config file must reference a [kubeconfig](/docs/concepts/cluster-administration/authenticate-across-clusters-kubeconfig/) formatted file which sets up the connection to the backend. It is required that the backend communicate over TLS.
 
 The kubeconfig file's cluster field must point to the remote service, and the user field must contain the returned authorizer.