Fix #5056 Document how stack uses Cabal-the-library

[mpilgrem]

The motivation for documenting this comes from investigating #5706 and #5707. It can be stated quite briefly, so I do not think it is out of place under the FAQ.

I also correct the incorrect guidance in the same place about stack build initialising a stack.yaml file.

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

Please also shortly describe how you tested your change. Bonus points for added tests! N/A

[andreasabel]

Thanks for clarifying the Cabal dependency in the docs!