doc/yaml_configuration.md: clarify setup-info behaviour

[borsboom]

@afcowie How does this clarification look?

[istathar]

That's great. Two suggestions:

• You might tweak it to say "if you need to replace it..."

• link the the canonical URL of the "default", being stack-setup-2.yaml (aka https://github.com/commercialhaskell/stackage-content/raw/master/stack/stack-setup-2.yaml as pulled by the stack binary. Figuring that path out [to subsequently patch it, see patch-setup] was a lot of work).

otherwise, many thanks.

AfC

[borsboom]

Alright, I've added another commit with your suggestions.