don't mention new rustdoc
don't mention new rustdoc

Steve says it's not ready
(https://github.com/rust-lang-nursery/rustc-guide/pull/150#issuecomment-395783504).

rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide