Document the RP2040-specific PS/2 implementation

Document the use of the PIO state machine code for RP2040 devices as an
alternative implementation of a PS/2 interface.

docs/feature_ps2_mouse.md

@@ -155,6 +155,27 @@ In your keyboard config.h:
 #endif
 ```
 
+### RP2040 PIO Version :id=rp2040-pio-version
+
+This uses the RP2040's hardware feature called PIO and is therefore only
+available on boards that are based on the RP2040 MCU.
+
+The pin for the data signal must be followed directly by the pin for the clock
+within the GPIO pin numbering scheme. This is required due to the design of the
+PIO state machine code.
+
+Example info.json content:
+
+```
+    "ps2": {
+        "clock_pin": "GP1",
+        "data_pin": "GP0",
+        "driver": "vendor",
+        "enabled": true,
+        "mouse_enabled": true
+    }
+```
+
 ## Additional Settings :id=additional-settings
 
 ### PS/2 Mouse Features :id=ps2-mouse-features

platforms/chibios/drivers/vendor/RP/RP2040/ps2_vendor.c

@@ -22,7 +22,7 @@
 #endif
 
 #if PS2_DATA_PIN + 1 != PS2_CLOCK_PIN
-#    error PS/2 Clock pin must be followed by data pin!
+#    error PS/2 data pin must be followed by clock pin!
 #endif
 
 static inline void pio_serve_interrupt(void);