feat(docs): Document new underglow Kconfig

docs/docs/config/underglow.md

@@ -14,9 +14,11 @@ RGB underglow depends on [Zephyr's LED strip driver](https://github.com/zephyrpr
 Definition file: [zmk/app/Kconfig](https://github.com/zmkfirmware/zmk/blob/main/app/Kconfig)
 
 | Config                                   | Type | Description                                               | Default |
-| ------------------------------------ | ---- | ----------------------------------------------------- | ------- |
+| ---------------------------------------- | ---- | --------------------------------------------------------- | ------- |
 | `CONFIG_ZMK_RGB_UNDERGLOW`               | bool | Enable RGB underglow                                      | n       |
 | `CONFIG_ZMK_RGB_UNDERGLOW_EXT_POWER`     | bool | Underglow toggling also controls external power           | y       |
+| `CONFIG_ZMK_RGB_UNDERGLOW_AUTO_OFF_IDLE` | bool | Turn off RGB underglow when keyboard goes into idle state | n       |
+| `CONFIG_ZMK_RGB_UNDERGLOW_AUTO_OFF_USB`  | bool | Turn off RGB underglow when USB is disconnected           | n       |
 | `CONFIG_ZMK_RGB_UNDERGLOW_HUE_STEP`      | int  | Hue step in degrees (0-359) used by RGB actions           | 10      |
 | `CONFIG_ZMK_RGB_UNDERGLOW_SAT_STEP`      | int  | Saturation step in percent used by RGB actions            | 10      |
 | `CONFIG_ZMK_RGB_UNDERGLOW_BRT_STEP`      | int  | Brightness step in percent used by RGB actions            | 10      |