[docs] [issue18841] math.isfinite fails with Decimal sNAN

[Serhiy Storchaka]

Changes by Serhiy Storchaka <storchaka at gmail.com>:


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nosy: +facundobatista, mark.dickinson, rhettinger, skrah

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue18841>
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