clarify the docs and new warning message.

Doc/library/subprocess.rst

@@ -160,7 +160,7 @@ This module defines one class called :class:`Popen`:
 
 .. versionchanged:: 3.2
    Callers should always specify a *close_fds* to avoid a DeprecationWarning.
-   The default value for this argument will be changing in Python 3.3.
+   The default behavior of this argument will be changing in Python 3.3.
 
    If *shell* is :const:`True`, the specified command will be executed through the
    shell.

Lib/subprocess.py

@@ -623,8 +623,8 @@ class Popen(object):
             # Notification for http://bugs.python.org/issue7213 & issue2320
             warnings.warn(
                     'The close_fds parameter was not specified.  Its default'
-                    ' will change from False to True in a future Python'
-                    ' version.  Most users should set it to True.  Please'
+                    ' behavior will change in a future Python version. '
+                    ' Most users should set it to True.  Please'
                     ' update your code explicitly set close_fds.',
                     DeprecationWarning)