man: describe how cgroup controllers are turned on

man: describe how cgroup controllers are turned on

For a user, information which cgroup controllers are enabled based on
the unit configuration is rather important. Not only because it determines
what resource control is peformed by the kernel, but also because controllers
have a non-negligible cost, especially for deep nesting, and users may want
to *not* have controllers enabled.

Our documentation did its best to avoid the topic so far. This was partially
caused by the support for cgroup v1, which meant that any discussion of
controllers had to be conditional and messy. But v1 is deprecated on its way
out, so it should be fine to just describe what happens with v2.

The text is extended with a discussion of how controllers are enabled and
disabled, and an example, and for various settings that enable controllers
the relevant controller is now mentioned.