[docs] [issue19789] Improve wording of how to "undo" a call to logging.disable(lvl)

[Ezio Melotti]

Changes by Ezio Melotti <ezio.melotti at gmail.com>:


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nosy: +vinay.sajip

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue19789>
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