fix(rgb): add missing Kconfig docs

docs/docs/feature/underglow.md

@@ -35,11 +35,17 @@ If your board or shield does not have RGB underglow configured, refer to [Adding
 
 There are various Kconfig options used to configure the RGB underglow feature. These can all be set in the `.conf` file.
 
-| Option                       | Description                                    | Default |
-| ---------------------------- | ---------------------------------------------- | ------- |
-| `ZMK_RGB_UNDERGLOW_HUE_STEP` | Hue step in degrees of 360 used by RGB actions | `10`    |
-| `ZMK_RGB_UNDERGLOW_SAT_STEP` | Saturation step in percent used by RGB actions | `10`    |
-| `ZMK_RGB_UNDERGLOW_BRT_STEP` | Brightness step in percent used by RGB actions | `10`    |
+| Option                        | Description                                    | Default |
+|-------------------------------|------------------------------------------------|---------|
+| `ZMK_RGB_UNDERGLOW_HUE_STEP`  | Hue step in degrees of 360 used by RGB actions | 10      |
+| `ZMK_RGB_UNDERGLOW_SAT_STEP`  | Saturation step in percent used by RGB actions | 10      |
+| `ZMK_RGB_UNDERGLOW_BRT_STEP`  | Brightness step in percent used by RGB actions | 10      |
+| `ZMK_RGB_UNDERGLOW_HUE_START` | Default hue 0-359 in degrees                   | 0       |
+| `ZMK_RGB_UNDERGLOW_SAT_START` | Default saturation 0-100 in percent            | 100     |
+| `ZMK_RGB_UNDERGLOW_BRT_START` | Default brightness 0-100 in percent            | 100     |
+| `ZMK_RGB_UNDERGLOW_SPD_START` | Default effect speed 1-5                       | 3       |
+| `ZMK_RGB_UNDERGLOW_EFF_START` | Default effect integer from the effect enum    | 0       |
+| `ZMK_RGB_UNDERGLOW_ON_START`  | Default on state                               | y       |
 
 ## Adding RGB Underglow to a Board