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 requires dates from a pre-PyPI & setuptools time and has now been replaced by install_requires.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)

Examinez la documentation existante sur les arguments de setup ainsi que le contexte de l’issue #264 avant de décider où ces notes doivent être placées. Documentez les précautions concernant requires, provides, obsoletes, platforms, dependency_links et download_url, en considérant le travail terminé lorsque les six arguments sont couverts.