db/DatabaseSong: DatabaseDetachSong(uri) returns instance, not pointer

src/db/DatabaseSong.hxx

@@ -37,12 +37,12 @@ DatabaseDetachSong(const Storage &storage, const LightSong &song);
 
 /**
  * Look up a song in the database and convert it to a #DetachedSong
- * instance.  The caller is responsible for freeing it.
+ * instance.
  *
- * @return nullptr on error
+ * Throws std::runtime_error on error.
  */
-gcc_malloc gcc_nonnull_all
-DetachedSong *
+gcc_pure
+DetachedSong
 DatabaseDetachSong(const Database &db, const Storage &storage,
 		   const char *uri);