scx_lavd: improve --help-stats option

[multics69]

Currently "--help-stats" prints only the the description of SysStats.
It should prints also SchedSample defined in stats.rs

$>scx_lavd  --help-stats                                                                                                                                                                                                                                                     
[SysStats]
  avg_svc_time   (u64)    : Average runtime per schedule
  mseq           (u64)    : Sequence ID of this messge
  nr_active      (u64)    : Number of active CPUs when core compaction is enabled
  nr_queued_task (u64)    : Number of runnable tasks in runqueues
  nr_sched       (u64)    : Number of context switches
  pc_balanced    (float)  : % of balanced mode
  pc_big         (float)  : % of tasks scheduled on big cores
  pc_greedy      (float)  : % of greedy tasks
  pc_lc          (float)  : % of latency-critical tasks
  pc_lc_on_big   (float)  : % of latency-critical tasks scheduled on big cores
  pc_lhp         (float)  : % lock holder preemption
  pc_migration   (float)  : % of task migration
  pc_pc          (float)  : % of performance-critical tasks
  pc_pc_on_big   (float)  : % of performance-critical tasks scheduled on big cores
  pc_performance (float)  : % of performance mode
  pc_powersave   (float)  : % of powersave powersave
  pc_preemption  (float)  : % of task preemption
  power_mode     (string) : Current power mode

[multics69]

Thanks @luigidematteis for the work.