Machine.DAC missing from ESP32 docs

Nothing in the machine module docs:
https://docs.micropython.org/en/latest/library/machine.html#module-machine

But it is there:
https://github.com/micropython/micropython/blob/master/ports/esp32/machine_dac.c

And it works:
```
from machine import Pin, DAC
dac = DAC(Pin(25))
for _ in range(100):
  for i in range(255):
    dac.write(i)
    dac.write(255-i)
```