policies/expression: add pb_flow_plan variable

docs/policies/expression/index.md

@@ -10,6 +10,7 @@ The following objects are passed into the variable:
     - `request.user`: The current User, which the Policy is applied against. ([ref](../../property-mappings/reference/user-object.md))
     - `request.http_request`: The Django HTTP Request, as documented [here](https://docs.djangoproject.com/en/3.0/ref/request-response/#httprequest-objects).
     - `request.obj`: A Django Model instance. This is only set if the Policy is ran against an object.
+- `pb_flow_plan`: Current Plan if Policy is called while a flow is active.
 - `pb_is_sso_flow`: Boolean which is true if request was initiated by authenticating through an external Provider.
 - `pb_is_group_member(user, group_name)`: Function which checks if `user` is member of a Group with Name `gorup_name`.
 - `pb_logger`: Standard Python Logger Object, which can be used to debug expressions.