lib: Make cross_toml module private

and don't add inline docs to the module. Instead, refer to the `docs/`
folder in the crate docs.

src/cross_toml.rs

@@ -1,5 +1,3 @@
-#![doc = include_str!("../docs/config_file.md")]
-
 use crate::docker::custom::PreBuild;
 use crate::docker::PossibleImage;
 use crate::shell::MessageInfo;

src/lib.rs

@@ -34,7 +34,7 @@ mod tests;
 pub mod cargo;
 pub mod cli;
 pub mod config;
-pub mod cross_toml;
+mod cross_toml;
 pub mod docker;
 pub mod errors;
 mod extensions;