[docs] [issue17188] Document 'from None' in raise statement doc.

[Arfrever Frehtes Taifersar Arahesis]

Changes by Arfrever Frehtes Taifersar Arahesis <Arfrever.FTA at GMail.Com>:


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nosy: +Arfrever

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue17188>
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