[docs] [issue21492] email.header.decode_header sometimes returns bytes, sometimes str

Change by Terry J. Reedy <tjreedy@udel.edu>: ---------- versions: +Python 3.7, Python 3.8, Python 3.9 -Python 3.3 _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue21492> _______________________________________