Document discouraged / deprecated setup arguments

We presently document an opinionated set of setup args that are applicable to most packaging scenarios. We also have additional topics for more advanced use cases. However, we don't presently document arguments that should never be used or used with extreme caution.

So far, the ones I'm aware of are:

1. requires dates from a pre-PyPI & setuptools time and has now been replaced by install_requires.
2. provides is unused.
3. obsoletes is unused.
4. platforms has ambiguous behavior.
5. dependency_links isn't supported by pip.
6. download_url has surprising behavior and shouldn't be used.

(Original context: #264)

Prüfe die vorhandene Dokumentation zu setup-Argumenten sowie den Kontext in Issue #264, bevor du entscheidest, wo diese Hinweise hingehören. Dokumentiere die Hinweise zu requires, provides, obsoletes, platforms, dependency_links und download_url; der Abschluss ist erreicht, wenn alle sechs Argumente abgedeckt sind.