Hidden type shows up in rustdoc

[jethrogb]

On https://doc.rust-lang.org/stable/std/cmp/trait.Ord.html and other pages, one might find implementors such as:

impl Ord for Big32x36
impl Ord for Big8x3

But these types are not exported and should be hidden from the documentation.

[hanna-kruppe]

Note: Technically these types are accessible — that is, with the proper #![feature()] you can use them — but they are inside a #[doc(hidden)] module and not re-exported from there.

The most serious part of the problem is that Big32x36, Big8x3, etc. are links to non-existing pages, (they 404).

[mitaa]

These aren't shown anymore since #31903