cgroup: Polish hierarchically aware protection docs a bit
    
    I missed adding a section in `systemd.resource-control` about
    DefaultMemoryMin in #12332.
    
    Also, add a NEWS entry going over the general concept.
    
    (cherry picked from commit acdb4b5236f38bbefbcc4a47fdbb9cd558b4b5c5)
    
    Related: #1763435
    
    patch_name: 0409-cgroup-Polish-hierarchically-aware-protection-docs-a.patch
    present_in_specfile: true
    location_in_specfile: 409
    squash_commits: true