cgroup, docs: Clarify limitation of RT processes with cgroup v2 cpu controller

The limitation that all RT processes have to be in the root cgroup
before enabling cpu controller only applies if the CONFIG_RT_GROUP_SCHED
option is enabled in the running kernel. If a kernel does not have
CONFIG_RT_GROUP_SCHED enabled, RT processes can exist in a non-root
cgroup even when cpu controller is enabled. CPU sharing of RT processes
will not be under cgroup control, but other resources like memory can be.

Clarify this limitation to avoid confusion to users that are using
cgroup v2.

Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>