[docs] [issue35083] Fix documentation for __instancecheck__

[Julien Palard]

Julien Palard <julien+python at palard.fr> added the comment:

Currently [1] there's no concensus about:

- Is this a documentation issue and we should document the optimisation?
- Is the optimisation over-zealeous?

[1]: https://mail.python.org/pipermail/python-ideas/2018-October/054342.html

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nosy: +mdk

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<https://bugs.python.org/issue35083>
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