[issue20803] doc: clarify that struct.pack_into writes 0x00 for pad bytes

[Diana]

Change by Diana <zemlya3018 at gmail.com>:


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keywords: +patch
pull_requests: +26493
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/28047

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<https://bugs.python.org/issue20803>
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