Document `pub` requirement for `new_without_default` lint

fixes rust-lang#8415

Also adds some UI tests that ensure that `pub` is required on both the struct _and_ the field. The only thing I'm not sure about is that the lint actually [checks](https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/new_without_default.rs#L102) if `new` is _reachable_, not _public_. To the best of my understanding, both the struct and the method need to be public for the method to be reachable for external crates (I certainly didn't manage to craft a counterexample).

changelog: Document `pub` requirement for ``[`new_without_default`]`` lint.