Document that setting sys.py3kwarning wont do anything.
diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index 8e6f399..9d33c60 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -624,7 +624,9 @@
.. data:: py3kwarning
Bool containing the status of the Python 3.0 warning flag. It's ``True``
- when Python is started with the -3 option.
+ when Python is started with the -3 option. (This should be considered
+ read-only; setting it to a different value doesn't have an effect on
+ Python 3.0 warnings.)
.. versionadded:: 2.6