[docs] [issue22374] Replace contextmanager example and improve explanation

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

swap_attr() looks too general. I think something more concrete would be better. But the documentation already contain other examples for contextmanager. closing() is good example, and redirect_stdout() would be good example.

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nosy: +serhiy.storchaka

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<http://bugs.python.org/issue22374>
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