I re-documented wrpc-dump, because I forgot I already did.
Still, this is a little better than what we had, and it's at the
same leve as SNMP, rather than relegated in an appendix.

So the new docs are committed, and the appendix just lists use of the
tool with old wrpc releases, which is not important to most.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>