allow(rustdoc::broken_intra_doc_links) not propagated to methods copied across due to Deref/DerefMut impl

[joshtriplett]

library/core/src/slice/mod.rs has this on binary_search_by_key:

    // Lint rustdoc::broken_intra_doc_links is allowed as `slice::sort_by_key` is
    // in crate `alloc`, and as such doesn't exists yet when building `core`: #74481.
    // This breaks links when slice is displayed in core, but changing it to use relative links
    // would break when the item is re-exported. So allow the core links to be broken for now.
    #[allow(rustdoc::broken_intra_doc_links)]

That fixes the issue for the documentation generated in that file. However, adding another type in a different file with a Deref to a slice (e.g. a Deref to [u8]) will result in broken intra-doc links for that file as well, and the allow won't be taken into account for those.

Using allow(rustdoc::broken_intra_doc_links) on a method should apply to any copies of that method propagated due to Deref/DerefMut.