[docs] [issue18713] Clearly document the use of PYTHONIOENCODING to set surrogateescape

[Serhiy Storchaka]

Changes by Serhiy Storchaka <storchaka at gmail.com>:


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dependencies: +Empty PYTHONIOENCODING is not the same as nonexistent

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue18713>
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