--before argument is currently described in ambiguous way: it
is not actually used to filter entries ARCHIVED before specified
date. Instead, it compares provided data with "deleted_at" value
for most rows and "updated_at" or "created_at" for remaining ones.
Since we already talk about time of deletion when describing
--before argument of "nova-manage db archive_deleted_rows" rows,
it make sense to not provide extra details here as well.
Change-Id: Ib5940e88a52dc8d32303e27237e567c3481fc3dc
ac8729ac87