Update docstring with more useful text (from the PEP)
diff --git a/Lib/numbers.py b/Lib/numbers.py
index 2592643..bdc6dd6 100644
--- a/Lib/numbers.py
+++ b/Lib/numbers.py
@@ -303,7 +303,7 @@
         raise NotImplementedError
 
     def __index__(self):
-        """index(self)"""
+        """Called whenever an index is needed, such as in slicing"""
         return long(self)
 
     @abstractmethod