[Distutils] dist-dir option missing in docs

[Luc Saffre]

Hi,

AFAICS the `dist-dir` option of the `setup.py sdist` command is not
documented in http://docs.python.org/distutils/sourcedist.html.
You might smile when hearing that I had to analyze the code to find out
that "python setup.py --help sdist" would have given me the answer.
I suggest at least simple screenshot of that command on the above page.

Best regards
Luc Saffre