Add RP2040 PIO UART implementation for split keyboards

docs/split_keyboards.md

@@ -41,6 +41,7 @@ split = Split(
     data_pin2=None,  # Second uart pin to allow 2 way communication
     target_left=True,  # Assumes that left will be the one on USB. Set to folse if it will be the right
     uart_flip=True,  # Reverses the RX and TX pins if both are provided
+    use_pio=False,  # Use RP2040 PIO implementation of UART. Required if you want to use other pins than RX/TX
 )
 
 ```