[docs] [issue12644] document the "%a" conversion in the old string formatting operations

[Éric Araujo]

Éric Araujo <merwok at netwok.org> added the comment:

I don’t think anyone could find anything wrong with the patch.

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stage: patch review -> commit review

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue12644>
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