Cleanup: Document use of index for wmGizmoPropertyFnForeachRNAProp

Pull Request: https://projects.blender.org/blender/blender/pulls/154551

source/blender/windowmanager/gizmo/wm_gizmo_fn.hh

@@ -73,6 +73,10 @@ using wmGizmoPropertyFnRangeGet = void (*)(const wmGizmo *,
 /**
  * To inspect the RNA properties gizmos are manipulating (can be multiple).
  * Used e.g. for auto-keying.
+ *
+ * \param gz_prop: The gizmo property to inspect.
+ * \param callback: Called for each RNA property being manipulated.
+ *        - `index`: Property element index (-1 for all items).
  */
 using wmGizmoPropertyFnForeachRNAProp =
     void (*)(wmGizmoProperty *,