cgroup: update PSI file description in docs

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: Odin Ugedal <odin@uged.al>
Acked-by: Dan Schatzberg <dschatzberg@fb.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Tejun Heo <tj@kernel.org>