Document (better) when/if `Setup.hs` is required

As far as I can tell, having a `Setup.hs` file is not required (if we have `build-type: Simple`, which is the default for newer cabal versions). However, [the documentation](https://cabal.readthedocs.io/en/stable/cabal-package-description-file.html#creating-a-package) doesn't mention this and instead makes it sound like it's required. Could you please clarify whether a `Setup.hs` file should be present and update the documentation in case it isn't?

Apologies if this was already answered, but there are too many issues involving `Setup.hs`.