document DefaultOOMPolicy
    
    the `man systemd.service` say:
       Defaults to the setting DefaultOOMPolicy= in systemd-system.conf(5) is set to
    but there is no such line in this config.
    This is the default value I extracted from
       systemctl show --property=DefaultOOMPolicy
    
    (cherry picked from commit 0084d4f6b58f70c9955fa2259c29d65d87e1e855)