In the documentation we usually spell the concept "control group".
Internally in code we usually call it "cgroup" or "CGroup". In systemctl output we
called the field "CGroup" so far, i.e. a capitalized version of the
internal name. This is of course very unsystematic. Let's clean this up
a bit: let's now say:
* in docs, continue to spell it out "control groups"
* in brief output call it "CGroup"
* internally call it "cgroup" or "CGroup"
Fixes: #14429