website/docs: add snippet to skip authenticated flow

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

website/docs/flow/examples/snippets.md

@@ -15,3 +15,11 @@ return False
 ```
 
 This policy should be bound to the stage after your redirect should happen. For example, if you have an identification and a password stage, and you want to redirect after identification, bind the policy to the password stage. Make sure the policy binding is set to re-evaluate policies.
+
+### Deny flow when user is authenticated
+
+```python
+return not request.user.is_authenticated
+```
+
+When used with authentik 2022.7 or later, set the flow _Denied action_ to _CONTINUE_. This will redirect already authenticated users to the default interface if they try to use the respective flow.