[docs] [issue35083] Fix documentation for __instancecheck__

[Karthikeyan Singaravelan]

Karthikeyan Singaravelan <tir.karthi at gmail.com> added the comment:

python-ideas thread for the issue : https://mail.python.org/pipermail/python-ideas/2018-October/054335.html

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nosy: +xtreak

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35083>
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