feat(docs): Document RGB momentary effect selection

docs/docs/behaviors/underglow.md

@@ -35,13 +35,15 @@ Here is a table describing the action for each define:
 | `RGB_SPD`       | Decreases the speed of the RGB feature effect's animation                                      |
 | `RGB_EFF`       | Cycles the RGB feature's effect forwards                                                       |
 | `RGB_EFR`       | Cycles the RGB feature's effect reverse                                                        |
+| `RGB_EFS`       | Selects a specific RGB effect                                                                  |
+| `RGB_MEFS`      | Selects a specific RGB effect whilst held down and reverts when released                       |
 | `RGB_COLOR_HSB` | Sets a specific [HSB (HSV)](https://en.wikipedia.org/wiki/HSL_and_HSV) value for the underglow |
 
 ## Behavior Binding
 
 - Reference: `&rgb_ug`
 - Parameter #1: The RGB action define, e.g. `RGB_TOG` or `RGB_BRI`
-- Parameter #2: Only applies to `RGB_COLOR_HSB` and is the HSB representation of the color to set (see below for an example)
+- Parameter #2: Applies to `RGB_EFS` and `RGB_MEFS` (the effect to select) as well as `RGB_COLOR_HSB` (the HSB representation of the color to set). See below for examples.
 
 :::note[HSB Values]
 
@@ -61,6 +63,18 @@ They will also override the start values set by [`CONFIG_ZMK_RGB_*_START` settin
 However the settings will only be saved after [`CONFIG_ZMK_SETTINGS_SAVE_DEBOUNCE`](../config/system.md#general) milliseconds in order to reduce potential wear on the flash memory.
 :::
 
+:::note Effect Selection
+
+When using the `RGB_EFS` or `RGB_MEFS` definitions you must also include a number as an argument in the keymap corresponding to the effect you want to select e.g. `RGB_EFS 0`
+
+:::
+
+:::warning
+
+If the `RGB_MEFS` key is held down for longer than [`CONFIG_ZMK_SETTINGS_SAVE_DEBOUNCE`](../config/system.md#general) milliseconds and the board is reset prior to releasing the key, the temporary effect will have been saved to flash memory and will be the one selected after resetting/power cycling.
+
+:::
+
 ## Examples
 
 1. Toggle underglow on/off
@@ -75,6 +89,12 @@ However the settings will only be saved after [`CONFIG_ZMK_SETTINGS_SAVE_DEBOUNC
    &rgb_ug RGB_COLOR_HSB(128,100,100)
    ```
 
+1. Select a specific RGB effect (Swirl)
+
+   ```dts
+   &rgb_ug RGB_EFS 3
+   ```
+
 ## Split Keyboards
 
 RGB underglow behaviors are global: This means that when triggered, they affect both the central and peripheral side of split keyboards.