sched_ext: Documentation: Update sched-ext.rst

    CONFIG_DEBUG_INFO_BTF=y

    CONFIG_BPF_JIT_ALWAYS_ON=y

    CONFIG_BPF_JIT_DEFAULT_ON=y

    CONFIG_PAHOLE_HAS_BTF_TAG=y

sched_ext is used only when the BPF scheduler is loaded and running.

  The functions prefixed with ``scx_bpf_`` can be called from the BPF

  scheduler.

* ``kernel/sched/ext_idle.c`` contains the built-in idle CPU selection policy.

* ``tools/sched_ext/`` hosts example BPF scheduler implementations.

  * ``scx_simple[.bpf].c``: Minimal global FIFO scheduler example using a

  * ``scx_qmap[.bpf].c``: A multi-level FIFO scheduler supporting five

    levels of priority implemented with ``BPF_MAP_TYPE_QUEUE``.

  * ``scx_central[.bpf].c``: A central FIFO scheduler where all scheduling

    decisions are made on one CPU, demonstrating ``LOCAL_ON`` dispatching,

    tickless operation, and kthread preemption.

  * ``scx_cpu0[.bpf].c``: A scheduler that queues all tasks to a shared DSQ

    and only dispatches them on CPU0 in FIFO order. Useful for testing bypass

    behavior.

  * ``scx_flatcg[.bpf].c``: A flattened cgroup hierarchy scheduler

    implementing hierarchical weight-based cgroup CPU control by compounding

    each cgroup's share at every level into a single flat scheduling layer.

  * ``scx_pair[.bpf].c``: A core-scheduling example that always makes

    sibling CPU pairs execute tasks from the same CPU cgroup.

  * ``scx_sdt[.bpf].c``: A variation of ``scx_simple`` demonstrating BPF

    arena memory management for per-task data.

  * ``scx_userland[.bpf].c``: A minimal scheduler demonstrating user space

    scheduling. Tasks with CPU affinity are direct-dispatched in FIFO order;

    all others are scheduled in user space by a simple vruntime scheduler.

ABI Instability

===============

The APIs provided by sched_ext to BPF schedulers programs have no stability

guarantees. This includes the ops table callbacks and constants defined in

``include/linux/sched/ext.h``, as well as the ``scx_bpf_`` kfuncs defined in

``kernel/sched/ext.c``.

``kernel/sched/ext.c`` and ``kernel/sched/ext_idle.c``.

While we will attempt to provide a relatively stable API surface when

possible, they are subject to change without warning between kernel