Stabilize checking of cfgs at compile-time: `--check-cfg` option This PR stabilize the `--check-cfg` CLI option of `rustc` (and `rustdoc`) 🎉. In particular this PR does two things: 1. it makes the `--check-cfg` option stable 2. and it moves the documentation to the stable books FCP: https://github.com/rust-lang/rust/issues/82450#issuecomment-1965328542 Resolves #82450 ``@rustbot`` labels +S-blocked +F-check-cfg r? ``@petrochenkov``