[Distutils] Distributing doc files w/ distutils src pkg?

[Skip Montanaro]

I have a simple setup.py file:

    #!/usr/bin/env python

    from distutils.core import setup
    setup(name='lockfile',
          author='Skip Montanaro',
          author_email='skip <at> pobox.com',
          maintainer='Skip Montanaro',
          maintainer_email='skip <at> pobox.com',
          url='http://www.webfast.com/~skip/python/',
          download_url='http://www.webfast.com/~skip/python/lockfile.tar.gz',
          version='0.1',
          py_modules=['lockfile'],
          data_files=[('doc', ['lockfile.rst'])],
          )

I've used the data_files arg to setup to note where the docs live.  Alas,
the sdist command doesn't include this in the tar file:

    % tar tfz dist/lockfile-0.1.tar.gz 
    lockfile-0.1/
    lockfile-0.1/lockfile.py
    lockfile-0.1/PKG-INFO
    lockfile-0.1/README
    lockfile-0.1/setup.py

I'm a complete distutils novice.  I can't for the life of me understand why
the doc directory isn't included in the tar file.  As far as I can tell
distutils has no explicit provision at all for distributing documentation,
which seems very weird.  What am I missing?