[docs] [issue19225] lack of PyExc_BufferError doc

[Kinebuchi Tomohiko]

Kinebuchi Tomohiko added the comment:

I will create a pull request based on the patch created by beng94.
In addition, I will have its contents up-to-date.

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versions: +Python 3.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue19225>
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