document-hidden-items should show when items are hidden

[Manishearth]

document-private-items shows a pub(crate) / empty on methods, and a 🔒 on types in the module interface.

We should do the same for hidden items with document-hidden-items, perhaps showing #[doc(hidden)] on methods and a 👻 in the types list.

Thoughts?

(n.b. document-hidden-items is currently -Zunstable-options)

[GuillaumeGomez]

I think it's a pretty good idea. 👍