[issue1410739] Add notes to the manual about `is` and methods

[Christian Heimes]

Christian Heimes added the comment:

The example sure is surprising for somebody without intimate knowledge
about Python's memory management. Although the is operator is
implemented as id(a) == id(b) reference counting, free lists and arenas
can cause some surprising effects.

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nosy: +tiran

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Tracker <report at bugs.python.org>
<http://bugs.python.org/issue1410739>
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