[Docs] Added note about no split support (#12512)

Currently, this feature isn't supported on splits, spent about an hour troubleshooting my board and firmware before asking, and turns out it's not in place. Note added to save others from this in the future.

docs/feature_led_indicators.md

@@ -1,5 +1,7 @@
 # LED Indicators
 
+?>  Currently, this feature is not supported for split keyboards
+
 QMK provides methods to read 5 of the LEDs defined in the HID spec:
 
 * Num Lock