I can't find documentation for extra_path anywhere.. but this is the 
documentation I found by searching google ( http://
mail.python.org/pipermail/distutils-sig/2000-March/000803.html ), 
from an old USAGE.txt that sits in the CVS attic now:

extra_path:
     information about extra intervening directories to put between
     'install_lib' and 'install_sitelib', along with a .pth file to
     ensure that those directories wind up in sys.path.  Can be a 1- 
or
     2-tuple, or a comma-delimited string with 1 or 2 parts.  The
     1-element case is simpler: the .pth file and directory have the 
same
    name (except for ".pth").  Eg. if extra_path is "foo" or ("foo",),
     then Distutils sets 'install_site_lib' to 'install_lib' +
     "site-packages/foo", and puts foo.path in the "site-packages"
     directory; it contains just "foo".  The 2-element case allows the
     .pth file and intervening directories to be named differently; 
eg.
    if 'extra_path' is ("foo","foo/bar/baz") (or "foo,foo/bar/baz"),
     then Distutils will set 'install_site_lib' to 'install_lib' +
     "site-packages/foo/bar/baz", and put "foo.pth" containing
     "foo/bar/baz" in the "site-packages" directory.