sched/rt/docs: Clarify & fix sched_rt_* sysctl docs

- Describe explicitly that sched_rt_runtime_us is allocated from
  sched_rt_period_us and hence always less or equal to that value.

- The limit for sched_rt_runtime_us is not INT_MAX-1, but rather it's
  limited by the value of sched_rt_period_us. If sched_rt_period_us is
  INT_MAX then sched_rt_runtime_us can be set to INT_MAX as well.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20231002115553.3007-3-chrubis@suse.cz