docs/sched_ext: document that cgroup CPU knobs are scheduler-dependent

The scheduler core communicates the initial cpu controller settings
to the BPF scheduler through ops.cgroup_init() and reports subsequent
changes through the corresponding ops.cgroup_set_*() callbacks.
Whether and how a knob takes effect is up to the loaded scheduler:
it may implement the corresponding callback partially or not at all,
so cpu.max, cpu.weight and friends can silently have no effect.

Document this in the basics section of sched-ext.rst.

Signed-off-by: Tao Cui <cuitao@kylinos.cn>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Documentation/scheduler/sched-ext.rst

@@ -242,6 +242,21 @@ optional. The following modified excerpt is from
             .name                   = "simple",
     };
 
+Scheduler-Dependent Knobs
+-------------------------
+
+The fair-class scheduler enforces CPU controller settings such as
+``cpu.max``, ``cpu.weight`` and ``cpu.idle``. For sched_ext tasks, the
+scheduler core communicates these settings to the BPF scheduler
+through ``ops.cgroup_init()`` and reports subsequent changes through
+the corresponding ``ops.cgroup_set_*()`` callbacks. Similarly, per-task
+nice changes are converted to weights and reported through
+``ops.set_weight()``.
+
+Each BPF scheduler is responsible for implementing the scheduling
+semantics of these settings and may choose to ignore them. Consult the
+loaded scheduler's documentation before relying on these controls.
+
 Dispatch Queues
 ---------------