NEGCACHE: Add some comments about each step of sss_ncache_prepopulate()
    
    The comments help to understand which part of the code is dealing with
    users or groups of specific or non-specific domain filters.
    
    Related: https://pagure.io/SSSD/sssd/issue/3460
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit b54d79cf3c8017e186b5ea7cdc383746233db39b)
    (cherry picked from commit 1d3d2a86b89aa3a24dbe8ffa770f1a28b645ee07)