Selecting MRF policies didn't work as intended any more

* Policies were put under a new module (Pleroma.Web.ActivityPub.MRF.Policy instead of Pleroma.Web.ActivityPub.MRF), but this wasn't changed in the Pleroma.Web.ActivityPub.MRF @mrf_config_descriptions
* I don't have a unit test to prevent similar problems in the future because I don't find a proper way to do it
    * The descriptions in the unit tests are defined in the unit tests, so if someone changes module names in the code, the tests wont see it
    * The list is generated in Pleroma.Docs.Generator.list_behaviour_implementations, but I can't do a check in the when clause of the function to see if the provided module is a behaviour or not.

Backport of: https://git.pleroma.social/pleroma/pleroma/-/merge_requests/3509

CHANGELOG.md

@@ -29,6 +29,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Mastodon API: Fix crash in Streamer related to reblogging
 - AdminAPI: List available frontends when `static/frontends` folder is missing
 - Make activity search properly use language-aware GIN indexes
+- AdminAPI: Fix suggestions for MRF Policies
 
 ## 2.4.0 - 2021-08-08
 

lib/pleroma/web/activity_pub/mrf.ex

@@ -21,7 +21,7 @@ defmodule Pleroma.Web.ActivityPub.MRF do
           type: [:module, {:list, :module}],
           description:
             "A list of MRF policies enabled. Module names are shortened (removed leading `Pleroma.Web.ActivityPub.MRF.` part), but on adding custom module you need to use full name.",
-          suggestions: {:list_behaviour_implementations, Pleroma.Web.ActivityPub.MRF}
+          suggestions: {:list_behaviour_implementations, Pleroma.Web.ActivityPub.MRF.Policy}
         },
         %{
           key: :transparency,

test/pleroma/docs/generator_test.exs

@@ -23,7 +23,7 @@ defmodule Pleroma.Docs.GeneratorTest do
           key: :filters,
           type: {:list, :module},
           description: "",
-          suggestions: {:list_behaviour_implementations, Pleroma.Web.ActivityPub.MRF}
+          suggestions: {:list_behaviour_implementations, Pleroma.Web.ActivityPub.MRF.Policy}
         },
         %{
           key: Pleroma.Upload,