Add a note about using rust-bindgen (#2818)

Co-authored-by: Dmitri Gribenko <[email protected]>

Author: JasonXJ

src/unsafe-rust/exercise.md

@@ -74,3 +74,13 @@ functions and methods:
 
 {{#include exercise.rs:main}}
 ```
+
+<details>
+
+FFI binding code is typically generated by tools like [bindgen], rather than
+being written manually as we are doing here. However, bindgen can't run in an
+online playground.
+
+</details>
+
+[bindgen]: https://github.com/rust-lang/rust-bindgen