Documentation needs to be improved around function of CAN vs. USB vs. HVIN

It is very unclear that you MUST use the CAN connector to power the board and cannot use the HVIN + USB connectors alone.

For future readers, here's how the various connectors work for real-world usage:

- If you're flashing firmware, you can connect just USB with the `VUSB` jumper in place. >>> _Be certain to remove the `VUSB` jumper before connecting any power or CAN cables_ <<<
- If using CAN as a communication interface and you don't need extra juice for the steppers, just connect the CAN plug
- Set each stepper voltage jumper to `VIN`
- If your mainboard supports CAN but you otherwise don't know what to do, this is probably the option you want
- If using CAN as a communication interface and you DO need extra power for the steppers, connect the CAN plug and also connect your additional power leads to the `HVIN` and `GND`
- Set each stepper voltage jumper to `HVIN`
- If using USB as a communication interface and you don't need extra juice for the steppers, you have to connect the CAN plug and put 12-24v through its power leads. You then have to connect a USB cable from your printer to the MMB
- Set each stepper voltage jumper to `VIN`
- If your mainboard DOES NOT support CAN but you otherwise don't know what to do, this is probably the option you want
- If using USB as a communication interface and you DO need extra power for the steppers, you have to connect the CAN plug and put 12-24v through its power leads and also connect your additional power leads to the `HVIN` and `GND`. You then have to connect a USB cable from your printer to the MMB
- Set each stepper voltage jumper to `HVIN`

The issue names no documentation file or test. Start by locating the board's hardware or wiring documentation entry point, then compare its CAN, USB, HVIN, GND, VUSB, and stepper-voltage-jumper guidance with the cases listed here. Done means the power and communication combinations are clearly documented, including the VUSB warning.

Written by the indexing model from the issue text.