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

[Ezio Melotti]

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


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keywords: +easy
nosy: +benjamin.peterson, ezio.melotti
type:  -> enhancement

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