docs(indev): add explanation for LV_INDEV_TYPE_BUTTON (#4478)

docs/porting/indev.rst

@@ -181,9 +181,12 @@ should look like ``const lv_point_t points_array[] = { {12,30},{60,90}, ...}``
           data->state = LV_INDEV_STATE_RELEASED; /*Set the released state*/
        }
 
-       data->btn = last_btn;            /*Save the last button*/
+       data->btn_id = last_btn;         /*Save the last button*/
    }
 
+When the ``button_read`` callback in the example above changes the ``data->btn_id`` to ``0``
+a press/release action at the first index of the ``points_array`` will be performed (``{12,30}``).
+
 Other features
 **************