cpuset: Add documentation about the new "cpuset.sched.partition" flag

The cgroup-v2.rst file is updated to document the purpose of the new
"cpuset.sched.partition" flag and how its usage.

Signed-off-by: Waiman Long <longman@redhat.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Tejun Heo <tj@kernel.org>