sched_ext: Documentation: Document events sysfs file and module parameters

sched_ext: Documentation: Document events sysfs file and module parameters

Two categories of sched_ext diagnostics are currently undocumented:

1. Per-scheduler events sysfs file
   Each active BPF scheduler exposes a set of diagnostic counters at
   /sys/kernel/sched_ext/<name>/events.  These counters are defined
   (with detailed comments) in kernel/sched/ext_internal.h but have
   no corresponding documentation in sched-ext.rst.  BPF scheduler
   developers must read kernel source to understand what each counter
   means.

   Add a description of the events file, an example of its output, and
   a brief explanation of every counter.

2. Module parameters
   kernel/sched/ext.c registers two parameters under the sched_ext.
   prefix (slice_bypass_us, bypass_lb_intv_us) via module_param_cb()
   with MODULE_PARM_DESC() strings, but sched-ext.rst makes no mention
   of them.  Users who need to tune bypass-mode behavior have no
   in-tree documentation to consult.

   Add a "Module Parameters" section documenting both knobs: their
   default values, valid ranges (taken from the set_*() validators in
   ext.c), and the note from the source that they are primarily for
   debugging.

No functional changes.

Signed-off-by: zhidao su <suzhidao@xiaomi.com>
Signed-off-by: Tejun Heo <tj@kernel.org>