[docs] [issue10289] Document magic methods called by built-in functions

[Serhiy Storchaka]

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


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nosy: +serhiy.storchaka
versions: +Python 3.6 -Python 3.4

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue10289>
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