[issue21358] Augmented assignment doc: clarify 'only evaluated once'

[Irit Katriel]

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


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versions: +Python 3.10, Python 3.11, Python 3.9 -Python 2.7, Python 3.4, Python 3.5

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue21358>
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