diff --git a/IPython/core/formatters.py b/IPython/core/formatters.py
index 60779f0..0f89332 100644
--- a/IPython/core/formatters.py
+++ b/IPython/core/formatters.py
@@ -60,7 +60,11 @@ class DisplayFormatter(Configurable):
             self.active_types = self.format_types
     
     active_types = List(Unicode, config=True,
-        help="""List of currently active mime-types""")
+        help="""List of currently active mime-types to display.
+        You can use this to set a white-list for formats to display.
+        
+        Most users will not need to change this value.
+        """)
     def _active_types_default(self):
         return self.format_types