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

[Terry J. Reedy]

Change by Terry J. Reedy <tjreedy at udel.edu>:


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versions: +Python 3.7, Python 3.8, Python 3.9 -Python 3.3

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue21492>
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