Document interaction with stackage-content

[dbaynard]

It would be helpful to explain this in terms of stack's platform support. I'm not yet sure where in the documentation it ought to go. probably in install_and_upgrade.md.

Related:

• Add comprehensive matrix of GHC bindists #3024
• commercialhaskell:stackage-content@3bbfe079501e9b1d47f8737ebbb5ecded8c5a780

[mpilgrem]

I am closing this issue because:

• setup-info and setup-info-locations document their defaults: https://docs.haskellstack.org/en/stable/yaml_configuration/#setup-info; and
• forthcoming global-hints-location will document its default (Fix #5638 Add global-hints-location option #6491)