[docs] [issue33187] Document ElementInclude (XInclude) support in ElementTree

[miss-islington]

Change by miss-islington <mariatta.wijaya+miss-islington at gmail.com>:


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pull_requests: +15593
pull_request: https://github.com/python/cpython/pull/15958

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue33187>
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