document power options

The pixhawk has a power management IC. The power supply is triple-redundant and will work for any combination of inputs on:

usb (required for comms)
power module input (required for power monitoring)
servo rail (required for camera tilt servo power, the other power inputs will power the cpu, but will not provide a servo power on this rail)
We recommend (and to a functional extent, require) using all three inputs.

No file or test is named. First locate the relevant ArduSub or Pixhawk power documentation page, then use the issue body as the source for the three inputs and their requirements. Done means the page clearly explains the triple-redundant supply, each input's role, and the recommendation to use all three.

Written by the indexing model from the issue text.