Mention that os.O_CLOEXEC was added to Python 3.3
diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 6adfdb1..b2a16d9 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -1369,6 +1369,8 @@
 
    These constants are only available on Unix.
 
+   .. versionchanged:: 3.3
+      Add :data:`O_CLOEXEC` constant.
 
 .. data:: O_BINARY
           O_NOINHERIT