sched/core: Update task_prio() function header

The description of the RT offset and the values for 'normal' tasks needs
update. Moreover there are DL tasks now.
task_prio() has to stay like it is to guarantee compatibility with the
/proc/<pid>/stat priority field:

  # cat /proc/<pid>/stat | awk '{ print $18; }'

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20210128131040.296856-4-dietmar.eggemann@arm.com

kernel/sched/core.c

@@ -5616,8 +5616,12 @@ SYSCALL_DEFINE1(nice, int, increment)
  * @p: the task in question.
  *
  * Return: The priority value as seen by users in /proc.
- * RT tasks are offset by -200. Normal tasks are centered
- * around 0, value goes from -16 to +15.
+ *
+ * sched policy         return value   kernel prio    user prio/nice
+ *
+ * normal, batch, idle     [0 ... 39]  [100 ... 139]          0/[-20 ... 19]
+ * fifo, rr             [-2 ... -100]     [98 ... 0]  [1 ... 99]
+ * deadline                     -101             -1           0
  */
 int task_prio(const struct task_struct *p)
 {