[ietf-dkim] Issue 1578: various minor documentation enhancements

[Stephen Farrell]

Issue description: https://rt.psg.com/Ticket/Display.html?id=1578

I didn't check that all these were done, but they're near-nits so
if there's no further discussion I'll ask Eliot to close this one
on July 11. (Dave - can you check that you're ok with ssp-04
for this one and let us know if there're any remaining TBD.)

S,