Clarify non_exhaustive behavior with unit structs

[ehuss]

The current page on #[non_exhaustive] doesn't really say much about unit structs. I think it could use some clarification. For example, the visibility of the unit struct const is lowered to pub(crate) like tuple struct constructors.

The RFC has more detail. It might be good to also comb the RFC and think about anything else missing from the non_exhaustive docs.