voltronic* documentation updates

 - Add to man page index
 - Reword a few sections
 - Fix typos
 - Comment out USB section in voltronic_ser.txt

Long-term, we should probably figure out a better way to maintain two parallel
driver pages like this. The blazer man page is the same for both, with
.so links for the man pages, but then you have USB info in a serial driver
page. For now, voltronic_usb.txt is just a copy of voltronic_ser.txt with a
few _ser-to-_usb replacements.