[docs] [issue14394] Add speed improvement note to the decimal docs.

[Raymond Hettinger]

Raymond Hettinger <raymond.hettinger at gmail.com> added the comment:

The correct place for the note is in the "optimizations" section of whatsnew.

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nosy: +rhettinger

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue14394>
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