clarify meaning of NFS port numbers in tcpdump.1

Update the text in the "NFS Requests and Replies" section to match the
changes made in commits 68fe98a, f3051bc and 29d83db, which make output
format different for UDP and TCP.

So long as the difference remains keep it documented and keep in mind
that even after it goes away the manpage should mention the change for
a while.