[docs] [issue25041] document AF_PACKET socket address format

[Mariatta Wijaya]

Changes by Mariatta Wijaya <mariatta.wijaya at gmail.com>:


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stage: patch review -> needs patch
versions: +Python 3.7 -Python 3.4, Python 3.5

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue25041>
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