SF bug 1071087: os.walk example for deleting a full tree is sometime wrong.

SF bug 1071087: os.walk example for deleting a full tree is sometime wrong.

Clarify that the example assumes no links are present; the point of the
example is to illustrate a need for topdown=False, not to wrestle with
platform-dependent link convolutions.

Also spell os.path.join() out in full, instead of using a shortcut import.
The bug reporter was confused by that too, and it's clearer this way.

Bugfix candidate; but I don't intend to backport it.