[docs] [issue13997] Clearly explain the bare minimum Python 3 users should know about Unicode

[Giampaolo Rodola']

Changes by Giampaolo Rodola' <g.rodola at gmail.com>:


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nosy: +giampaolo.rodola

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue13997>
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