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

[Nitika Agarwal]

Changes by Nitika Agarwal <nitikaagarwal18 at gmail.com>:


Added file: http://bugs.python.org/file34434/issue17188.patch

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