PWM/DAC API Update

* Improves documentation around PWM.
* Removes documentation around the DAC until we fix its issues.

Do not merge until https://github.com/tessel/firmware/pull/7 is merged.
if you call pwmDutyCycle without having called pwmFrequency, you get an Error `PWM is not configured. Call `port.pwmFrequency(freq)` first.`