[issue13814] Document why generators don't support the context management protocol

[Irit Katriel]

Change by Irit Katriel <iritkatriel at gmail.com>:


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keywords: +easy -patch
versions: +Python 3.10, Python 3.11, Python 3.9 -Python 3.5, Python 3.6

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue13814>
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