doc: kernel: threads: fix idle thread priority documentation

Clarify that the idle thread has priority CONFIG_NUM_PREEMPT_PRIORITIES,
not (CONFIG_NUM_PREEMPT_PRIORITIES - 1).

The idle thread priority ranges from 0 to 127 and never has a cooperative
thread priority.

Signed-off-by: Jonas Spinner <[email protected]>

Author: jonas-5ae29c