[docs] [issue17902] Document that _elementtree C API cannot use custom TreeBuilder for iterparse or IncrementalParser

[Eli Bendersky]

Changes by Eli Bendersky <eliben at gmail.com>:


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resolution:  -> fixed
stage: patch review -> committed/rejected
status: open -> closed

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue17902>
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