Force PWM mode for power supply on GPIO, or suggest it in documentation

[tfry-git]

          And, I wanted to add that with Mozzi version 2, PWM output ( custom pin) if I use the second cpu I can use the adafruit libraries and I2C bus for display and muxed input on the RP2040 (a pico). To reduce noise, don't forget:

  pinMode(23, OUTPUT); // thi is to switch to PWM for power to avoid ripple noise
  digitalWrite(23, HIGH);

which keeps the pico from dropping into PFM power saving mode and forces PWM mode. That's quiet. Running from a 3.7 volt lithium with the display active and a simple RC output, it's really nice! I have to see what, if anything is possible with the beloved nano :)

Originally posted by @poetaster in #209 (comment)