release candidate SOP: update args, improve some comments
    
    The main change here is to update the args for the script to be
    correct after https://pagure.io/pungi-fedora/pull-request/1369 .
    Also, we explain that the compose request might include commands
    for untagging/tagging builds (this feature was added a few months
    ago), and extend/clarify the comments on the label format.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>