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

[Antoine Pitrou]

Changes by Antoine Pitrou <pitrou at free.fr>:


----------
nosy: +eli.bendersky
stage:  -> patch review
versions: +Python 3.3

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