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

[Aaron Oakley]

Aaron Oakley added the comment:

So sorry, I just found the emails from the bug tracker in my spam folder. Anyhow, I've now signed the CLA.

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