stable rustdoc should prune unstable items from stdlib docs

[tomjakubowski]

For example, http://doc.rust-lang.org/std/error/trait.Error.html includes impls for unstable structs like std::ffi::IntoStringError. Like #31098, this is "dangling a carrot" which users of stable rust can't reach! I think that the doc page for IntoStringError shouldn't exist in the set of stable docs.

There are already passes to strip things like #[doc(hidden)] and private items from the crate before generating documentation. The same could be done for unstable items.

[tomjakubowski]

Okay, I see this is a dupe of #25863. I don't think it's true that the set of unstable items is small, but I'll close this issue.