Add description in Bluetooth docs for requiring NKRO to be disabled (#10359)

docs/feature_bluetooth.md

@@ -26,7 +26,10 @@ A Bluefruit UART friend can be converted to an SPI friend, however this [require
 
 <!-- FIXME: Document bluetooth support more completely. -->
 ## Bluetooth Rules.mk Options
-Use only one of these
+
+The currently supported Bluetooth chipsets do not support [N-Key Rollover (NKRO)](reference_glossary.md#n-key-rollover-nkro), so `rules.mk` must contain `NKRO_ENABLE = no`.
+
+Use only one of these to enable Bluetooth:
 * BLUETOOTH_ENABLE = yes (Legacy Option)
 * BLUETOOTH = RN42
 * BLUETOOTH = AdafruitBLE