feat(object_tree): make lv_obj_get_index return error value if the object has no parent (#4152)

src/core/lv_obj_tree.h

@@ -151,7 +151,8 @@ uint32_t lv_obj_get_child_cnt(const struct _lv_obj_t * obj);
  * Get the index of a child.
  * @param obj       pointer to an object
  * @return          the child index of the object.
- *                  E.g. 0: the oldest (firstly created child)
+ *                  E.g. 0: the oldest (firstly created child).
+ *                  (0xFFFFFFFF if child could not be found or no parent exists)
  */
 uint32_t lv_obj_get_index(const struct _lv_obj_t * obj);