Backport #10833: Complete the documentation of main-is field

[mergify]

The motivation is the discussion at this Stack issue:

• Cabal's main-is field (source file) does not alter GHC's -main-is option (module and function name) commercialhaskell/stack#6695

where a user wanted to use Foo.hs containing Foo.main and expected Cabal's main-is field to interpret that.

Explains the use of GHC's -main-is option in Cabal files, when the usual convention of Main.main is not being followed.

This PR does not modify behaviour or interface

• Patches conform to the coding conventions.
  Not applicable, but style of surrounding text is followed.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).
  Not applicable.

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This is an automatic backport of pull request #10833 done by [Mergify](https://mergify.com).