rustdoc: Add impl docs to demo mod

src/rustdoc/demo.rs

@@ -167,4 +167,15 @@ iface the_shunned_house {
         failure = "Will fail if bodies are removed from premises"
     )]
     fn construct() -> bool;
+}
+
+#[doc = "Whatever"]
+impl of the_shunned_house for omnomnomy {
+    #[doc(args(_unkempt_yard = "Whatever"))]
+    fn dingy_house(_unkempt_yard: int) {
+    }
+
+    fn construct() -> bool {
+        fail;
+    }
 }