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 requires dates from a pre-PyPI & setuptools time and has now been replaced by install_requires.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

Review the existing documentation for setup arguments and the context in issue #264 before deciding where these notes belong. Document the cautions for requires, provides, obsoletes, platforms, dependency_links, and download_url, with completion marked by all six arguments being covered.