examples/rp2: Add comment that examples using IO25 don't work on Pico W.

Signed-off-by: Damien George <damien@micropython.org>

examples/rp2/pio_1hz.py

@@ -1,4 +1,5 @@
 # Example using PIO to blink an LED and raise an IRQ at 1Hz.
+# Note: this does not work on Pico W because it uses Pin(25) for LED output.
 
 import time
 from machine import Pin

examples/rp2/pio_exec.py

@@ -1,4 +1,5 @@
 # Example using PIO to turn on an LED via an explicit exec.
+# Note: this does not work on Pico W because it uses Pin(25) for LED output.
 #
 # Demonstrates:
 #   - using set_init and set_base

examples/rp2/pwm_fade.py

@@ -1,4 +1,5 @@
 # Example using PWM to fade an LED.
+# Note: this does not work on Pico W because it uses Pin(25) for LED output.
 
 import time
 from machine import Pin, PWM