Update PSI file description in cgroup-v2 docs to reflect the current
implementation.

tj: Changed cpu.pressure from read-only to read-write as suggested by
    Johannes.
Signed-off-by: NOdin Ugedal <odin@uged.al>
Acked-by: NDan Schatzberg <dschatzberg@fb.com>
Acked-by: NJohannes Weiner <hannes@cmpxchg.org>
Signed-off-by: NTejun Heo <tj@kernel.org>