Clarify PIO define is optional in docs. (qmk#22339)

docs/serial_driver.md

@@ -235,7 +235,7 @@ Where 'n' matches the peripheral number of your selected USART on the MCU.
 
 The `PIO` subsystem is a Raspberry Pi RP2040 specific implementation, using the integrated PIO peripheral and is therefore only available on this MCU. Because of the flexible nature of the PIO peripherals, **any** GPIO pin can be used as a `TX` or `RX` pin. Half-duplex and Full-duplex operation is fully supported. The Half-duplex operation mode uses the built-in pull-ups and GPIO manipulation on the RP2040 to drive the line high by default. An external pull-up is therefore not necessary.
 
-Configure the hardware via your config.h:
+You may optionally switch the PIO peripheral used with the following define in config.h:
 ```c
 #define SERIAL_PIO_USE_PIO1 // Force the usage of PIO1 peripheral, by default the Serial implementation uses the PIO0 peripheral
 ```