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 requires dates from a pre-PyPI & setuptools time and has now been replaced by install_requires.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

これらの注記をどこに記載するかを決める前に、setup引数に関する既存のドキュメントと issue #264 のコンテキストを確認してください。requires、provides、obsoletes、platforms、dependency_links、download_url に関する注意事項を記載し、6つすべての引数を扱った時点で完了とします。