test(embedded): Note our lack of rust grammar parsing

Author: epage

src/cargo/util/toml/embedded.rs

@@ -381,6 +381,7 @@ fn main() {}
 
     #[test]
     fn rustc_frontmatter_after_tokens() {
+        // Deferred to rustc since this requires knowledge of Rust grammar
         assert_source(
             r#"#![feature(frontmatter)]