[Docs] Add reference to data sync options for RGB (#16144)

docs/feature_rgb_matrix.md

@@ -849,6 +849,8 @@ void rgb_matrix_indicators_advanced_user(uint8_t led_min, uint8_t led_max) {
 }
 ```
 
+!> RGB indicators on split keyboards will require state information synced to the slave half (e.g. `#define SPLIT_LAYER_STATE_ENABLE`). See [data sync options](feature_split_keyboard.md#data-sync-options) for more details.
+
 #### Indicators without RGB Matrix Effect
 
 If you want to just use RGB indicators without RGB matrix effect, it is not possible to disable the latter because toggling RGB off will disable everything. You can workaround it with solid effect and colors off using this init function:

docs/feature_rgblight.md

@@ -323,6 +323,8 @@ void post_process_record_user(uint16_t keycode, keyrecord_t *record) {
 ```
 would turn the layer 0 (or 1) on and off again three times when `DEBUG` is pressed.
 
+!> Lighting layers on split keyboards will require layer state synced to the slave half (e.g. `#define SPLIT_LAYER_STATE_ENABLE`). See [data sync options](feature_split_keyboard.md#data-sync-options) for more details.
+
 ### Overriding RGB Lighting on/off status
 
 Normally lighting layers are not shown when RGB Lighting is disabled (e.g. with `RGB_TOG` keycode). If you would like lighting layers to work even when the RGB Lighting is otherwise off, add `#define RGBLIGHT_LAYERS_OVERRIDE_RGB_OFF` to your `config.h`.