rustdoc: indicate when a trait has non-defaulted hidden items

[GoldsteinE]

There’s a pattern of hiding a non-defaulted item in a trait to mark it as non-implementable, e.g. inventory::Collect:

https://github.com/dtolnay/inventory/blob/5e915d4d197432b73375ad21e08a1ee8275d8309/src/lib.rs#L224-L227

This is primarily useful when a trait is implemented by a macro, so it can’t be properly sealed, but still needs to be only implementable by code “originating” in the same crate. It has some similarity with #[non_exhaustive] attribute.

Some options on how this could look: