[docs] [issue38843] Document argparse behaviour when custom namespace object already has the field set

Change by miss-islington <mariatta.wijaya+miss-islington@gmail.com>: ---------- nosy: +miss-islington nosy_count: 4.0 -> 5.0 pull_requests: +22535 pull_request: https://github.com/python/cpython/pull/23668 _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue38843> _______________________________________