Sealed traits for rustdoc

[Mark-Simulacrum]

In #67562, it was proposed to make trait Any an unsafe trait. This was ultimately rejected, as we do not currently want downstream users to see that it is unsafe. However, we wanted a way to have both. The suggestion is that rustdoc could have an (unstable, for now) attribute that indicates that a trait is "sealed," in the sense that no upstream impls are permitted.

rustdoc would then show this trait as unimplementable. It would not show any unsafety on the trait (as it is then irrelevant to readers of the documentation).

See #67562 (comment), #67562 (comment).