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

Change by Raymond Hettinger <raymond.hettinger@gmail.com>: ---------- keywords: +patch pull_requests: +22521 stage: -> patch review pull_request: https://github.com/python/cpython/pull/23653 _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue38843> _______________________________________