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

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