pkg/tinyusb: update doc for tusb_app_config.h

With PR RIOT-OS#18804 the approach to override the default tinyUSB configuration was changed. The update of the documentation was forgotten.

pkg/tinyusb/doc.txt

@@ -51,9 +51,9 @@
  * }
  * ```
  *
- * Create a file `tinyusb_app_config.h` in your application directory.
- * This file is can be used to override the default configuration defined
- * in `tusb_config.h`.
+ * If it is necessary to override the default configuration defined in
+ * `tusb_config.h`, create a file `tinyusb_app_config.h` in your application
+ * directory and override the configuration to be changed.
  * ```c
  * #define CONFIG_TUSBD_CDC_NUMOF   2
  * ```
@@ -68,7 +68,9 @@
  * enabled, `CONFIG_TUSBD_CDC_NUMOF` is defined to 1 by default. The number of
  * all other `CONFIG_TUSBD_*_NUMOF` device class interfaces are 0.
  *
- * Add the application path to the include paths at the end of your
+ * If you add the `tinyusb_app_config.h` file in your application directory
+ * to override the default configuration, add the application path to the
+ * include paths at the end of your
  * application's Makefile. This is necessary so that the tinyUSB stack
  * uses the file `tinyusb_app_config.h` from your application directory
  * and thus the file `tusb_config.h` from the tinyUSB package.