rustdoc: `doc(hidden)` methods from traits show up on implementors

[jsha]

The __iterator_get_unchecked method on Iterator is doc(hidden): https://doc.rust-lang.org/nightly/src/core/iter/traits/iterator.rs.html#3425-3437, and it is correctly hidden in the main part of Iterator's page. However, it still shows up in the Implementations section of structs that implement the trait, like std::str::Bytes:

https://doc.rust-lang.org/1.52.0/std/str/struct.Bytes.html#method.__iterator_get_unchecked

The method should be hidden in all contexts.

[jsha]

Looking at implementations of __iterator_get_unchecked in std, it looks like about half have doc(hidden) on the implementation too. So for now I'm going to fix my immediate issue by making sure all implementations have doc(hidden). Maybe that's the overall solution too - say that doc(hidden) only applies to the trait item, and if implementations want to hide their copy, they have to do that manually.

[jyn514]

Duplicate of #36035