Update prune docs in archiver.py and a few things in usage.rst.

I created a long epilog for the prune command.  This automatically shows up in usage.html via usage/prune.rst.inc.
I wasn't sure what to do about the existing "Description" for prune in usage.rst, which is now a bit redundant.  If all of the Descriptions get moved into epilogs, then all of the Descriptions could just be removed.  But the epilogs aren't formatted as nicely as the Descriptions...