another cStringIO restriction
diff --git a/Doc/library/stringio.rst b/Doc/library/stringio.rst
index f7f6771..30a5bac 100644
--- a/Doc/library/stringio.rst
+++ b/Doc/library/stringio.rst
@@ -72,8 +72,9 @@
instead.
Since this module provides a factory function which returns objects of built-in
-types, there's no way to build your own version using subclassing. Use the
-original :mod:`StringIO` module in that case.
+types, there's no way to build your own version using subclassing. It's not
+possible to set attributes on it. Use the original :mod:`StringIO` module in
+those cases.
Unlike the memory files implemented by the :mod:`StringIO` module, those
provided by this module are not able to accept Unicode strings that cannot be