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>

Author: odinuge

Documentation/admin-guide/cgroup-v2.rst

@@ -1029,7 +1029,7 @@ All time durations are in microseconds.
 	one number is written, $MAX is updated.
 
   cpu.pressure
-	A read-only nested-key file which exists on non-root cgroups.
+	A read-write nested-keyed file.
 
 	Shows pressure stall information for CPU. See
 	:ref:`Documentation/accounting/psi.rst <psi>` for details.
@@ -1475,7 +1475,7 @@ PAGE_SIZE multiple when read back.
 	reduces the impact on the workload and memory management.
 
   memory.pressure
-	A read-only nested-key file which exists on non-root cgroups.
+	A read-only nested-keyed file.
 
 	Shows pressure stall information for memory. See
 	:ref:`Documentation/accounting/psi.rst <psi>` for details.
@@ -1714,7 +1714,7 @@ IO Interface Files
 	  8:16 rbps=2097152 wbps=max riops=max wiops=max
 
   io.pressure
-	A read-only nested-key file which exists on non-root cgroups.
+	A read-only nested-keyed file.
 
 	Shows pressure stall information for IO. See
 	:ref:`Documentation/accounting/psi.rst <psi>` for details.