add docs

docs/feature_rgb_matrix.md

@@ -775,6 +775,7 @@ These are defined in [`color.h`](https://github.com/qmk/qmk_firmware/blob/master
 #define RGB_MATRIX_DISABLE_KEYCODES // disables control of rgb matrix by keycodes (must use code functions to control the feature)
 #define RGB_MATRIX_SPLIT { X, Y } 	// (Optional) For split keyboards, the number of LEDs connected on each half. X = left, Y = Right.
                               		// If RGB_MATRIX_KEYPRESSES or RGB_MATRIX_KEYRELEASES is enabled, you also will want to enable SPLIT_TRANSPORT_MIRROR
+#define RGB_TRIGGER_ON_KEYDOWN      // Triggers RGB keypress events on key down. This makes RGB control feel more responsive. This may cause RGB to not function properly on some boards
 ```
 
 ## EEPROM storage :id=eeprom-storage