Complete the documentation of main-is field

[mpilgrem]

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.

[ulysses4ever]

That's an improvement, thanks! Could you replace - with proper em-dashes if you really want dashes? Dash is a little tricky, and I'd simply use a comma or full stop depending on the context, but it's up to you. Also 'main' with single quotes looks strange to my eye --- I'd use double quotes or the code backticks. But again, it's a slippery place.

[mpilgrem]

@ulysses4ever, to avoid some long sentences, I've taken a slightly different approach. I've added two short sentences of introductory context, boiled down from the introduction to Chapter 5 of the Haskell Language Report, except that I have used the phrase "IO action" rather than "computation of type IO τ for some type τ". (While "IO action" could be said to be "Haskell jargon", I think it is a term that is encountered early on when you learn about Haskell's approach to I/O - and you have to learn that approach to write a Haskell program.) This allows me to refer to 'the convention' and 'the relevant IO action' later in the text.

[ulysses4ever]

Thanks for addressing the comments! I went ahead and put the merge label.

[ulysses4ever]

we may even consider backporting it to 3.14...