Remove todo for autogenerated modules

[philderbeast]

Fixes #11389, add documentation to the autogen-modules field and edit the existing explanation and warning.

---

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

[philderbeast]

Thanks for the grammar checks @geekosaur.

[geekosaur]

I should note that the original version of that warning isn't any better, so possibly some research is needed to clarify it.

[geekosaur]

I'm finding this warning a little difficult to follow. Does it intend to say that autogen-modules can't be used with a custom main-is, or that the module in main-is can't be an autogenerated module, or maybe something else?

[philderbeast]

I moved that warning from:

https://github.com/haskell/cabal/blame/9ea037b084125ecdd024aad02565b34b9db8316f/doc/cabal-package-description-file.rst#L3039-L3040

I've checked and it looks like there is not a cabal-testsuite test for the main-is warning (whichever way it goes).

[geekosaur]

Right, I saw the original (see the comment above it, I forgot the actual review summary goes above the review comments). Maybe leave as is for now and open an issue specifically about that warning, since it would be good to say exactly what the problem is instead of something vague and possibly misleading. I've fixed a few such in GHC NOTEs but would need to study the code to see what's up here..