[docs] [issue15561] update subprocess docs to reference io.TextIOWrapper

Chris Jerdonek added the comment:

As I can see in subprocess.py TextIOWrapper is applied to stdin also in non-buffered (write_through=True) mode.

In 3.2, I will not mention the write_through argument based on Antoine's response to issue 15638. ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue15561> _______________________________________