Validating that votes only have a single authority entry

Vote documents should only contain an entry from the authority that issued it.
I was checking this in my consensus-checker monitor but Karsten made the good
point that the check belongs in stem.
Parse exit list entry.

Published by DNSEL or TorBEL to indicate what ip address exit relay X
had at timestamp Y.

See #8255
Minor formatting changes for DNSEL addition

Nothing functional, just rearranging a few things to satisfy my OCD. :)
Adding TorDNSEL docs to the site