add explanation next to `#[cfg(TODO)]`s in code

[Lotterleben]

so that participants wondering why they can't instantiate a thing get a hint right where the problem is (I think it's mentioned in the book already)

[japaric]

I think it may be best to use block comments (/* .. */) which is more familiar to C and non-Rust programmers.

[Lotterleben]

Agreed. maybe we can sneak in a "🔎 in Rust projects, you may encounter another block comment notation: #[cfg(TODO)] effectively comments out the following element, which could be anything from a struct member to a module" in the materials?