docs: cabal.project search path is not documented

My understanding is that cabal.project affects a build even if the file lives in a parent directory, not only the current directory. Maybe this is well-known, but not for me, and I've been experiencing spooky actions at a distance for a little while. The closest the docs get to is (if I'm not mistaken):

> The full configuration of a project is determined by combining the following sources (later entries override earlier ones, except for appendable options):
>
> - ~/.cabal/config (the user-wide global configuration)
>
> - cabal.project (the project configuration)
>
> - cabal.project.freeze (the output of cabal v2-freeze)

Which, if anything, suggests that the global config file and the two from the current directory affect the build. At least, having an absolute path in the first bullet point indicates to me that the rest are probably relative paths.

If you agree that it's an omission, I could supply some kind of amendment to the above text.