`missing-docs-allow-unused` doesn't catch associated `const`s.

Summary

I suggested missing-docs-allow-unused in #14413 which has been useful, but it seems it was a bit constrained in its scope and only applies to fields. However I ran into the case today where the missing-docs-allow-unused behaved unexpectedly - it didn't silence an error in a const _SIZE_TEST: () = { assert!(...) } declaration where I'd have expected it to.

Could it be expanded to include any declaration that has an identifier that is _-prefixed?

Lint Name

missing_docs_in_private_items

Reproducer

I tried this code:

//! Test
pub struct Foo(pub usize);

impl Foo {
    const _SIZE_ASSERT: () = {
        assert!(
            core::mem::size_of::<Self>().is_multiple_of(4),
            "not a multiple of 4"
        );
    };
}

# clippy.toml
missing-docs-allow-unused = true

I saw this happen:

error: missing documentation for an associated constant
  --> src/lib.rs:5:5
   |
 5 | /     const _SIZE_ASSERT: () = {
 6 | |         assert!(
 7 | |             core::mem::size_of::<Self>().is_multiple_of(4),
 8 | |             "not a multiple of 4"
 9 | |         );
10 | |     };
   | |______^

I expected no clippy lint to emit.

Version

clippy 0.1.90 (1159e78c47 2025-09-14)

Additional Labels

No response