Add CORS note to docs

docs/admin/06_storage.rst

@@ -28,6 +28,12 @@ AlekSIS allows you to configure an Amazon S3 endpoint for  media
 files. This is useful e.g. for loadbalancing with multiple AlekSIS
 instances.
 
+.. note::
+   For some background jobs, AlekSIS stores HTML snippets in the media
+   storage for later use. You must ensure your S3 endpoint is part of
+   your ``Access-Control-Allow-Origin`` CORS header, so HTML loaded from
+   there can load resources from the ALekSIS instance.
+
 Configure an S3 endpoint
 ~~~~~~~~~~~~~~~~~~~~~~~~