system_api: return error on repeated registration of shutdown handler

components/esp_common/include/esp_system.h

@@ -80,6 +80,11 @@ typedef void (*shutdown_handler_t)(void);
   *
   * This function allows you to register a handler that gets invoked before
   * the application is restarted using esp_restart function.
+  * @param handle function to execute on restart
+  * @return
+  *   - ESP_OK on success
+  *   - ESP_ERR_INVALID_STATE if the handler has already been registered
+  *   - ESP_ERR_NO_MEM if no more shutdown handler slots are available
   */
 esp_err_t esp_register_shutdown_handler(shutdown_handler_t handle);