Doc fix for What's New page re. bytes_to_native_str

Thanks to @Veedrac for his pull request #47

Author: edschofield

docs/whatsnew.rst

@@ -247,8 +247,8 @@ Many small improvements and fixes have been made across the project. Some highli
 
 - Improved compatibility with py2exe (`issue #31 <https://github.com/PythonCharmers/python-future/issues/31>`_).
 
-- The ``future.utils.bytes_to_native_str`` function now returns a ``native_str``
-  object and ``future.utils.native_str_to_bytes`` returns a ``newbytes`` on Py2.
+- The ``future.utils.bytes_to_native_str`` function now returns a platform-native string
+  object and ``future.utils.native_str_to_bytes`` returns a ``newbytes`` object on Py2.
   (`Issue #47 <https://github.com/PythonCharmers/python-future/issues/47>`_).
 
 - The backported ``http.client`` module and related modules use other new