[issue3865] explain that profilers should be used for profiling, not benchmarking

[Terry J. Reedy]

Changes by Terry J. Reedy <tjreedy at udel.edu>:


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versions: +Python 2.7, Python 3.1, Python 3.2 -Python 2.6

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue3865>
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