Document *all* supported keywords, including those from distutils

[pganssle]

When setuptools was first developed and its documentation was first written, it was assumed that you were already using distutils and that you would have read and understood the distutils documentation, which is why setuptools only documents additional keywords, not all the supported options.

However, distutils is deprecated now, and and the distutils documentation is all in legacy mode, plus it's super annoying when you don't know which one contains the keywords you care about.

To satisfy this ticket, we should reorganize the documentation in the following ways:

1. Move the keywords documentation to their own separate documentation page.
2. Start moving all the documentation from the distutils. This will be slow going, I imagine, because there does not seem to be a "keyword reference" on distutils. I think you'll have to extract a lot of the values from this prose description, or possibly read the distutils source code itself.