Rollup merge of #101006 - GuillaumeGomez:doc-cfg-reexport, r=notriddle

Fix doc cfg on reexports

Fixes #83428.

The problem was that the newly inlined item cfg propagation was not working since its real parent is different than its current one.

For the implementation, I decided to put it directly into `CfgPropagation` instead of inside `inline.rs` because I thought it would be simpler to maintain and to not forget if new kind of items are added if it's all done in one place.

r? `@notriddle`

src/librustdoc/html/render/mod.rs

@@ -516,7 +516,14 @@ fn portability(item: &clean::Item, parent: Option<&clean::Item>) -> Option<Strin
         (cfg, _) => cfg.as_deref().cloned(),
     };
 
-    debug!("Portability {:?} - {:?} = {:?}", item.cfg, parent.and_then(|p| p.cfg.as_ref()), cfg);
+    debug!(
+        "Portability {:?} {:?} (parent: {:?}) - {:?} = {:?}",
+        item.name,
+        item.cfg,
+        parent,
+        parent.and_then(|p| p.cfg.as_ref()),
+        cfg
+    );
 
     Some(format!("<div class=\"stab portability\">{}</div>", cfg?.render_long_html()))
 }