rustdoc-search: add unbox flag to Result aliases

Fixes #139665

tests/rustdoc-js-std/unbox-type-result.js

@@ -0,0 +1,20 @@
+// exact-check
+
+// Test case for https://github.com/rust-lang/rust/issues/139665
+// make sure that std::io::Result and std::thread::Result get unboxed
+
+const EXPECTED = [
+    {
+        query: "File -> Metadata",
+        others: [
+            { path: "std::fs::File", name: "metadata" },
+            { path: "std::fs::File", name: "metadata_at" },
+        ]
+    },
+    {
+        query: "JoinHandle<T> -> T",
+        others: [
+            { path: "std::thread::JoinHandle", name: "join" },
+        ]
+    },
+];