[issue35714] Document that the null character '\0' terminates a struct format spec

[miss-islington]

Change by miss-islington <mariatta.wijaya+miss-islington at gmail.com>:


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nosy: +miss-islington
nosy_count: 6.0 -> 7.0
pull_requests: +19636
pull_request: https://github.com/python/cpython/pull/20373

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35714>
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