Rollup merge of rust-lang#125454 - mu001999-contrib:master, r=oli-obk

Improve the doc of query associated_item

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This query also maps from a impl item to the impl item "descriptor". So it's a bit confused, I skipped it cause it doesn't say it contains impl items.

```rust
fn associated_item(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::AssocItem {
    let id = tcx.local_def_id_to_hir_id(def_id);
    let parent_def_id = tcx.hir().get_parent_item(id);
    let parent_item = tcx.hir().expect_item(parent_def_id.def_id);
    match parent_item.kind {
        hir::ItemKind::Impl(impl_) => {
            if let Some(impl_item_ref) = impl_.items.iter().find(|i| i.id.owner_id.def_id == def_id)
            {
                let assoc_item = associated_item_from_impl_item_ref(impl_item_ref);
                debug_assert_eq!(assoc_item.def_id.expect_local(), def_id);
                return assoc_item;
            }
        }

        hir::ItemKind::Trait(.., trait_item_refs) => {
            if let Some(trait_item_ref) =
                trait_item_refs.iter().find(|i| i.id.owner_id.def_id == def_id)
            {
                let assoc_item = associated_item_from_trait_item_ref(trait_item_ref);
                debug_assert_eq!(assoc_item.def_id.expect_local(), def_id);
                return assoc_item;
            }
        }

        _ => {}
    }

    span_bug!(
        parent_item.span,
        "unexpected parent of trait or impl item or item not found: {:?}",
        parent_item.kind
    )
}
```

compiler/rustc_middle/src/query/mod.rs

@@ -780,7 +780,7 @@ rustc_queries! {
         separate_provide_extern
     }
 
-    /// Maps from a trait item to the trait item "descriptor".
+    /// Maps from a trait/impl item to the trait/impl item "descriptor".
     query associated_item(key: DefId) -> ty::AssocItem {
         desc { |tcx| "computing associated item data for `{}`", tcx.def_path_str(key) }
         cache_on_disk_if { key.is_local() }