Clarify role of ~/.cabal when determining config file.

[athas]

This is purely a documentation change that makes the behaviour more explicit.

The actual behaviour is unchanged, although the need for a five step checklist to find the config file suggests that perhaps things have gotten a bit out of hand.

Template B: This PR does not modify behaviour or interface

E.g. the PR only touches documentation or tests, does refactorings, etc.

Include the following checklist in your PR:

• 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).

[Mikolaj]

Python is acting up. Let me restart the failed CI jobs.

[Mikolaj]

Thank you.

[ulysses4ever]

Formally speaking, this should close #10713, which was a purely documentation inquiry. But I see an active discussion there, so up to you...

[geekosaur]

This is explicitly a follow-on to that issue, and is not a final solution as discussion continues about possibly simplifying the behavior.

[ulysses4ever]

@athas merge label?

[ulysses4ever]

all right, I'll take the liberty to put the merge label...

[ulysses4ever]

actually, @Bodigrim, can you take a look first? It was your issue, so it'd be good to have your opinion here.

[athas]

I don't understand what is expected of me here. I think this should be merged; otherwise I would have made the PR a draft.

[ulysses4ever]

@athas as per https://github.com/haskell/cabal/blob/master/CONTRIBUTING.md#github-pull-request-conventions the PR author is expected to put one of the merge labels when they feel like the PR should be merged.