little changes for MRF config descriptions

- log level reduction, if policy doesn't implement config_description method
- docs in dev.md

lib/pleroma/web/activity_pub/mrf.ex

@@ -164,7 +164,7 @@ defmodule Pleroma.Web.ActivityPub.MRF do
           acc
         end
       else
-        Logger.info(
+        Logger.debug(
           "#{policy} is excluded from config descriptions, because does not implement `config_description/0` method."
         )