Merge from 3.5
diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 3d03153..925a722 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -1804,6 +1804,8 @@
    This function can also support :ref:`paths relative to directory descriptors
    <dir_fd>`.
 
+   Availability: Unix.
+
    .. versionadded:: 3.3
       The *dir_fd* argument.