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.
+1 on it defaulting to something. IMO it should be a frequency in which you can see an LED blink on and off (this could be 980Hz). That's the first thing I'd do with a new PWM function to test if it's working or not.