cgroup: Fix documentation for cpu.idle

Two problems:
	- cpu.idle cgroups show up with 0 weight, correct the
	  documentation to indicate this.
	- cpu.idle has no entry describing it.
Signed-off-by: Josh Don <joshdon@google.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Documentation/admin-guide/cgroup-v2.rst

@@ -1093,7 +1093,11 @@ All time durations are in microseconds.
 	A read-write single value file which exists on non-root
 	cgroups.  The default is "100".
 
-	The weight in the range [1, 10000].
+	For non idle groups (cpu.idle = 0), the weight is in the
+	range [1, 10000].
+
+	If the cgroup has been configured to be SCHED_IDLE (cpu.idle = 1),
+	then the weight will show as a 0.
 
   cpu.weight.nice
 	A read-write single value file which exists on non-root
@@ -1157,6 +1161,16 @@ All time durations are in microseconds.
         values similar to the sched_setattr(2). This maximum utilization
         value is used to clamp the task specific maximum utilization clamp.
 
+  cpu.idle
+	A read-write single value file which exists on non-root cgroups.
+	The default is 0.
+
+	This is the cgroup analog of the per-task SCHED_IDLE sched policy.
+	Setting this value to a 1 will make the scheduling policy of the
+	cgroup SCHED_IDLE. The threads inside the cgroup will retain their
+	own relative priorities, but the cgroup itself will be treated as
+	very low priority relative to its peers.
+
 
 
 Memory