Update comment

roles/matrix-synapse/defaults/main.yml

@@ -158,9 +158,9 @@ matrix_synapse_container_additional_volumes: []
 # Contains definition objects like this: `{"name": "..", "level": "DEBUG"}
 matrix_synapse_additional_loggers: []
 
-# A list of service config files
+# A list of appservice config files (in-container filesystem paths).
 # This list gets populated dynamically based on Synapse extensions that have been enabled.
-# Contains fs paths
+# You may wish to use this together with `matrix_synapse_container_additional_volumes`.
 matrix_synapse_app_service_config_files: []
 
 # This is set dynamically during execution depending on whether