[docs] argparse: nargs='*' doesn't get out-of-order positional parameters (issue 14191)

Reviewers: , Message: This looks good to me, and definitely solves the problem I reported in the bug. Wish I'd had time learn the code and to do a nice thorough job of fixing the bug as paul.j3 did. In reviewing his changes, and integrating them in to my working version (where they are running fine), I did wind up learning a bit more than I had figured out on my own. Please review this at http://bugs.python.org/review/14191/ Affected files: Doc/library/argparse.rst Lib/argparse.py Lib/test/test_argparse.py