[docs] [issue13875] Improve description of cmd module

[Éric Araujo]

Changes by Éric Araujo <merwok at netwok.org>:


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resolution: works for me -> 
stage:  -> needs patch
title: cmd: no user documentation -> Improve description of cmd module

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue13875>
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