[docs] Documentation: argparse's actions store_{true, false} default to False/True (undocumented) (issue 25314)

https://bugs.python.org/review/25314/diff/16913/Doc/library/argparse.rst File Doc/library/argparse.rst (right): https://bugs.python.org/review/25314/diff/16913/Doc/library/argparse.rst#new... Doc/library/argparse.rst:924: keyword argument must be given. For other actions, is defaults to ``None``. Should be “it defaults to None” https://bugs.python.org/review/25314/