Auto merge of #82641 - camelid:lang-item-docs, r=jyn514

Improve lang item generated docs

cc https://rust-lang.zulipchat.com/#narrow/stream/146229-wg-secure-code/topic/Is.20.60core.60.20part.20of.20the.20compiler.3F/near/226738260

r? `@jyn514`

compiler/rustc_hir/src/lib.rs

@@ -5,6 +5,7 @@
 #![feature(crate_visibility_modifier)]
 #![feature(const_fn)] // For the unsizing cast on `&[]`
 #![feature(const_panic)]
+#![feature(extended_key_value_attributes)]
 #![feature(in_band_lifetimes)]
 #![feature(once_cell)]
 #![feature(or_patterns)]