sched_ext: Documentation: Remove mentions of scx_bpf_switch_all

If a task explicitly sets its scheduling policy to ``SCHED_EXT``, it will be

treated as ``SCHED_NORMAL`` and scheduled by CFS until the BPF scheduler is

loaded. On load, such tasks will be switched to and scheduled by sched_ext.

loaded.

The BPF scheduler can choose to schedule all normal and lower class tasks by

calling ``scx_bpf_switch_all()`` from its ``init()`` operation. In this

case, all ``SCHED_NORMAL``, ``SCHED_BATCH``, ``SCHED_IDLE`` and

``SCHED_EXT`` tasks are scheduled by sched_ext. In the example schedulers,

this mode can be selected with the ``-a`` option.

When the BPF scheduler is loaded and ``SCX_OPS_SWITCH_PARTIAL`` is not set

in ``ops->flags``, all ``SCHED_NORMAL``, ``SCHED_BATCH``, ``SCHED_IDLE``, and

``SCHED_EXT`` tasks are scheduled by sched_ext.

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 CFS.

Terminating the sched_ext scheduler program, triggering :kbd:`SysRq-S`, or

detection of any internal error including stalled runnable tasks aborts the

programs that implement ``struct sched_ext_ops``. The only mandatory field

is ``ops.name`` which must be a valid BPF object name. All operations are

optional. The following modified excerpt is from

``tools/sched/scx_simple.bpf.c`` showing a minimal global FIFO scheduler.

``tools/sched_ext/scx_simple.bpf.c`` showing a minimal global FIFO scheduler.

.. code-block:: c

            scx_bpf_dispatch(p, SCX_DSQ_GLOBAL, SCX_SLICE_DFL, enq_flags);

    }

    s32 BPF_STRUCT_OPS(simple_init)

    s32 BPF_STRUCT_OPS_SLEEPABLE(simple_init)

    {

            /*

             * All SCHED_OTHER, SCHED_IDLE, and SCHED_BATCH tasks should

             * use sched_ext.

             * By default, all SCHED_EXT, SCHED_OTHER, SCHED_IDLE, and

             * SCHED_BATCH tasks should use sched_ext.

             */

            scx_bpf_switch_all();

            return 0;

    }