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

Eli Bendersky added the comment: Aaron - could you describe your use case of passing a custom parser into iterparse? We're currently considering deprecating the feature of passing a parser into iterparse in a future release (this is being discussed in issue 17741). ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue17902> _______________________________________