chore: add comments to build script

bevy_lint/build.rs

@@ -3,14 +3,19 @@ use toml_edit::DocumentMut;
 const RUST_TOOLCHAIN: &str = include_str!("../rust-toolchain.toml");
 
 fn main() {
+    // Only re-run this build script if its source or `rust-toolchain.toml` was modified.
     println!("cargo::rerun-if-changed=build.rs");
     println!("cargo::rerun-if-changed=../rust-toolchain.toml");
 
     let rust_toolchain = RUST_TOOLCHAIN
         .parse::<DocumentMut>()
         .expect("Failed to parse `rust-toolchain.toml`.");
 
-    let channel = rust_toolchain["toolchain"]["channel"].as_str().unwrap();
+    let channel = rust_toolchain["toolchain"]["channel"]
+        .as_str()
+        .expect("Could not find `toolchain.channel` key in `rust-toolchain.toml`.");
 
+    // Emit the toolchain channel as an environmental variable that the crate can access using the
+    // `env!()` macro.
     println!("cargo::rustc-env=RUST_TOOLCHAIN_CHANNEL={channel}");
 }