docs: add a comment clarifying weird choice

Author: KSXGitHub

src/app/sub.rs

@@ -160,8 +160,7 @@ where
         if !only_one_arg {
             *data_tree.name_mut() = OsStringDisplay::os_string_from("(total)");
             if let Some(map) = &mut leaf_color_map {
-                let total = OsString::from("(total)");
-                let empty = OsString::from("");
+                // NOTE: cannot use for-in because we are modifying the keys
                 *map = map
                     .drain()
                     .map(|(mut key, color)| {