Auto merge of #5399 - dwijnand:cleanup/drop-an-old-visible-doc-todo, r=matklad

Drop an old visible doc TODO

Looks like this originates from
https://github.com/istankovic/cargo-book/commit/418611304daac22e3590aee9322355ae159802fe

But it's even visible in the render docs:
https://doc.rust-lang.org/cargo/reference/manifest.html#package-selection
:-O