Update prune documentation to use new command line options,
and also to say that the time that matters is the time each
backup completes.
When -v is specified with prune, also show the timestamp of each
archive, which is helpful for debugging.  Unify this with the
formatting done by "attic list".  The spacing is chosen so that
the attic prune output is 79 characters wide when the archive
names are at most 36 characters.
Merge remote-tracking branch 'jdchristensen/prune-docs-info'
testsuite: Fix test test_prune_repository