sched_ext: Documentation: Mention scheduling class precedence

Mention the scheduling class precedence of fair and sched_ext to
clear up how sched_ext partial mode works.

Signed-off-by: Christian Loehle <christian.loehle@arm.com>
Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: cloehle

Documentation/scheduler/sched-ext.rst

@@ -58,7 +58,8 @@ in ``ops->flags``, all ``SCHED_NORMAL``, ``SCHED_BATCH``, ``SCHED_IDLE``, and
 However, when the BPF scheduler is loaded and ``SCX_OPS_SWITCH_PARTIAL`` is
 set in ``ops->flags``, only tasks with the ``SCHED_EXT`` policy are scheduled
 by sched_ext, while tasks with ``SCHED_NORMAL``, ``SCHED_BATCH`` and
-``SCHED_IDLE`` policies are scheduled by the fair-class scheduler.
+``SCHED_IDLE`` policies are scheduled by the fair-class scheduler which has
+higher sched_class precedence than ``SCHED_EXT``.
 
 Terminating the sched_ext scheduler program, triggering `SysRq-S`, or
 detection of any internal error including stalled runnable tasks aborts the