[docs] [issue30969] Docs should say that `x is z or x == z` is used for `x in y` in containers that do not implement `__contains__`

[Antti Haapala]

Changes by Antti Haapala <antti at haapala.name>:


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pull_requests: +2820

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue30969>
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