[docs] [issue13770] python3 & json: add ensure_ascii documentation

[Ezio Melotti]

Changes by Ezio Melotti <ezio.melotti at gmail.com>:


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nosy: +ezio.melotti
versions:  -Python 3.1

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue13770>
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