expand active_types help string

IPython/core/formatters.py

@@ -60,7 +60,11 b' 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