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

[Eli Bendersky]

Eli Bendersky <eliben at gmail.com> added the comment:

%r has note (5) saying "The precision determines the maximal number of characters used."

Does this apply to %a as well?

Other than that, LGTM

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<http://bugs.python.org/issue12644>
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