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.
Thanks, I'll correct the naming. 

In my opinion, there should be a default pwm frequency because for many users, they won't even care what it is. Arduino defaults to 980Hz and nobody seems to mind. If folks have a specific application, they'll go looking for how to set the frequency.