Missing documentation of conditionals?

I couldn't find any mention of conditionals (`if`) in the [doc index](https://cabal.readthedocs.io/en/3.4/genindex.html), nor could I get help on possible values of the `os` predicate.

[`cabal check` alerts me that none of my tries works: `Warning: Unknown operating system name 'macosx', 'mac', 'macos'`]

Start with the Cabal documentation index and the behavior reported by `cabal check`. Determine where conditional expressions and the `os` predicate are documented, then document the supported operating-system values and conditional syntax. Done means the relevant documentation is discoverable from the index and matches the values accepted by `cabal check`.

Written by the indexing model from the issue text.