DOCS/interface-changes.rst: document loadfile breaking change

DOCS/interface-changes.rst

@@ -64,7 +64,11 @@ Interface changes
     - change `--hidpi-window-scale` default to `no`
     - add `insert-next`, `insert-next-play`, `insert-at`, and `insert-at-play`
       actions to `loadfile` and `loadlist` commands
-    - add `index` argument to `loadfile` and `loadlist` commands
+    - add `index` argument to `loadlist` command
+    - add `index` argument to `loadfile` command. This breaks all existing
+      uses of this command which make use of the argument to include the list of
+      options to be set while the file is playing. To address this problem, the
+      third argument now needs to be set to -1 if the fourth argument needs to be used.
     - move the `options` argument of the `loadfile` command from the third
       parameter to the fourth (after `index`)
     - add `--drag-and-drop=insert-next` option