Clarify request-branch 'service levels' argument
    
    Description of 'request-branch' command wasn't clear for '--sl' argument
    and was modified. '--sl' argument allows multiple values and therefore
    only positional argument 'branch' shouldn't be placed at the end of
    line.
    
    JIRA: COMPOSE-2970
    Fixes: #283
    Signed-off-by: Ondrej Nosek <onosek@redhat.com>