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

[Roundup Robot]

Roundup Robot added the comment:

New changeset a5a5ba4f71ad by Eli Bendersky in branch '3.3':
Issue #17902: Clarify doc of ElementTree.iterparse
http://hg.python.org/cpython/rev/a5a5ba4f71ad

New changeset 96f45011957e by Eli Bendersky in branch 'default':
Issue #17902: Clarify doc of ElementTree.iterparse and IncrementalParser
http://hg.python.org/cpython/rev/96f45011957e

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nosy: +python-dev

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