docs/scheduler: fix unit error

The unit mentioned in the documentation of scheduler statistics is
outdated which may mislead the readers.

The unit of statistics that is reported by /proc/schedstat is modified
to nanosecond, and the unit of statistics that is reported by
/proc/PID/schedstat is provided as well to make the context consistent.

The rq_cpu_time and the rq_sched_info.run_delay of a run queue, and the
sched_info.run_delay of a task are all updated based on the clock of the
run queue, while the se.sum_exec_runtime of a task is updated based on
the clock_task of the run queue of the task. Both the clock and
clock_task are relied on the return value of the function sched_clock()
which is in the unit of nanosecond.

Signed-off-by: Jui-Tse Huang <juitse.huang@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>