[docs] [issue16954] Add docstrings for ElementTree module

[Serhiy Storchaka]

Change by Serhiy Storchaka <storchaka+cpython at gmail.com>:


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stage: patch review -> needs patch
versions: +Python 3.7, Python 3.8 -Python 3.4

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue16954>
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