use _add_doc
diff --git a/six.py b/six.py
index c3f0c05..6bc97c4 100644
--- a/six.py
+++ b/six.py
@@ -500,10 +500,12 @@
     def iterlists(d, **kw):
         return iter(d.iterlists(**kw))
 
-iterkeys.__doc__ = """Return an iterator over the keys of a dictionary."""
-itervalues.__doc__ = """Return an iterator over the values of a dictionary."""
-iteritems.__doc__ = """Return an iterator over the (key, value) pairs of a dictionary."""
-iterlists.__doc__ = """Return an iterator over the (key, [values]) pairs of a dictionary."""
+_add_doc(iterkeys, "Return an iterator over the keys of a dictionary.")
+_add_doc(itervalues, "Return an iterator over the values of a dictionary.")
+_add_doc(iteritems,
+         "Return an iterator over the (key, value) pairs of a dictionary.")
+_add_doc(iterlists,
+         "Return an iterator over the (key, [values]) pairs of a dictionary.")
 
 
 if PY3: