Add test for `--document-hidden-items`

tests/rustdoc/issue-81141-private-reexport-in-public-api-hidden.rs

@@ -0,0 +1,16 @@
+// compile-flags: -Z unstable-options --document-hidden-items
+
+#![crate_name = "foo"]
+
+#[doc(hidden)]
+pub use crate::bar::Bar as Alias;
+
+mod bar {
+    pub struct Bar;
+}
+
+// @has 'foo/fn.bar.html'
+// @has - '//*[@class="rust item-decl"]/code' 'pub fn bar() -> Alias'
+pub fn bar() -> Alias {
+    Alias
+}