Update workers.md to make media_repository work (again) (#6519)

Author: mnowiasz

docs/workers.md

@@ -196,7 +196,7 @@ Handles the media repository. It can handle all endpoints starting with:
 
     /_matrix/media/
 
-And the following regular expressions matching media-specific administration APIs:
+... and the following regular expressions matching media-specific administration APIs:
 
     ^/_synapse/admin/v1/purge_media_cache$
     ^/_synapse/admin/v1/room/.*/media$
@@ -206,6 +206,18 @@ You should also set `enable_media_repo: False` in the shared configuration
 file to stop the main synapse running background jobs related to managing the
 media repository.
 
+In the `media_repository` worker configuration file, configure the http listener to
+expose the `media` resource. For example:
+
+```yaml
+    worker_listeners:
+     - type: http
+       port: 8085
+       resources:
+         - names:
+           - media
+```
+
 Note this worker cannot be load-balanced: only one instance should be active.
 
 ### `synapse.app.client_reader`