doc: Update LDAP grace period design with default values
    
    New group password policies will get -1 (unlimited) on creation
    by default.
    
    Existing group password policies will remain untouched and
    those created prior will be treated as no BIND allowed.
    
    Fixes: https://pagure.io/freeipa/issue/9212
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>