Fix setting values of 0 in ACME pruning
    
    Replace comparisons of "if value" with "if value is not None"
    in order to handle 0.
    
    Add a short reference to the man page to indicat that a cert
    or request retention time of 0 means remove at the next
    execution.
    
    Also indicate that the search time limit is in seconds.
    
    Fixes: https://pagure.io/freeipa/issue/9325
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Francisco Trivino <ftrivino@redhat.com>