Update common_settings.py to improve docs string.

models/common_settings.py

 # -*- coding: utf-8 -*-
 """ Common settings
 
+Model instantiate the database table as well as the UI configuration.
+It is tune to only satisfy the need of the controller, function pair.
+
+Note:
+    The model expose to controllers several global variables:
+
+        * auth
+        * directSvc
+
 """
 import plugin_dbui as dbui