Add test case for rust-lang#65863

Author: jyn514

src/test/rustdoc/return-impl-trait.rs

@@ -0,0 +1,15 @@
+#![feature(type_alias_impl_trait)]
+
+pub trait Backend {}
+
+impl Backend for () {}
+
+pub struct Module<T>(T);
+
+pub type BackendImpl = impl Backend;
+
+// @has return_impl_trait/fn.make_module.html
+/// Documentation
+pub fn make_module() -> Module<BackendImpl> {
+    Module(())
+}