Update docs to suggest adding kani to list of known configs

[zhassan-aws]

Code that includes #[cfg(kani)] in crates that use the nightly-2024-05-05 toolchain or later (or using the yet-to-be-released Rust v1.80), will get a warning. Here's a reproducer:

$ cargo new cfg-warn
     Created binary (application) `cfg-warn` package
$ cd cfg-warn/
$ echo "nightly-2024-05-21" > rust-toolchain

Add the following to src/main.rs:

fn main() { }

#[cfg(kani)]
mod kani_checks {
    #[kani::proof]
    fn check() {
        assert_eq!(1, 1);
    }
}

$ cargo build
info: syncing channel updates for 'nightly-2024-05-21-x86_64-unknown-linux-gnu'
info: latest update on 2024-05-21, rust version 1.80.0-nightly (b92758a9a 2024-05-20)
info: downloading component 'cargo'
info: downloading component 'clippy'
info: downloading component 'rust-docs'
info: downloading component 'rust-std'
info: downloading component 'rustc'
info: downloading component 'rustfmt'
info: installing component 'cargo'
info: installing component 'clippy'
info: installing component 'rust-docs'
info: installing component 'rust-std'
info: installing component 'rustc'
info: installing component 'rustfmt'
   Compiling cfg-warn v0.1.0 (/home/ubuntu/examples/cfg-warn)
warning: unexpected `cfg` condition name: `kani`
 --> src/main.rs:3:7
  |
3 | #[cfg(kani)]
  |       ^^^^
  |
  = help: expected names are: `clippy`, `debug_assertions`, `doc`, `docsrs`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, `windows`
  = help: consider using a Cargo feature instead
  = help: or consider adding in `Cargo.toml` the `check-cfg` lint config for the lint:
           [lints.rust]
           unexpected_cfgs = { level = "warn", check-cfg = ['cfg(kani)'] }
  = help: or consider adding `println!("cargo::rustc-check-cfg=cfg(kani)");` to the top of the `build.rs`
  = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
  = note: `#[warn(unexpected_cfgs)]` on by default

warning: `cfg-warn` (bin "cfg-warn") generated 1 warning
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.10s

We should update our docs to suggest adding kani to the list of known configurations in the Cargo.toml file if cfg(kani) is used. For example, adding the following will silence the warning:

[lints.rust]
unexpected_cfgs = { level = "warn", check-cfg = ['cfg(kani)'] }

$ cargo build
   Compiling cfg-warn v0.1.0 (/home/ubuntu/examples/cfg-warn)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.09s

Link to relevant documentation (Rust reference, Nomicon, RFC):

https://github.com/rust-lang/rust/blob/b92758a9aef1cef7b79e2b72c3d8ba113e547f89/src/doc/rustc/src/check-cfg/cargo-specifics.md#check-cfg-in-lintsrust-table