book: Clean up the C++ section some more
book: Clean up the C++ section some more

Trying to set expectations for what we can do with C++ a little better.

emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen