[Docs] Add RGB Matrix default mode define info (#6564)

docs/feature_rgb_matrix.md

@@ -374,6 +374,7 @@ These are defined in [`rgblight_list.h`](https://github.com/qmk/qmk_firmware/blo
 #define RGB_MATRIX_LED_PROCESS_LIMIT (DRIVER_LED_TOTAL + 4) / 5 // limits the number of LEDs to process in an animation per task run (increases keyboard responsiveness)
 #define RGB_MATRIX_LED_FLUSH_LIMIT 16 // limits in milliseconds how frequently an animation will update the LEDs. 16 (16ms) is equivalent to limiting to 60fps (increases keyboard responsiveness)
 #define RGB_MATRIX_MAXIMUM_BRIGHTNESS 200 // limits maximum brightness of LEDs to 200 out of 255. If not defined maximum brightness is set to 255
+#define RGB_MATRIX_STARTUP_MODE RGB_MATRIX_CYCLE_LEFT_RIGHT // Sets the default mode, if none has been set
 ```
 
 ## EEPROM storage