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 <jonas.spinner@burkert.com>

doc/kernel/services/threads/system_threads.rst

@@ -35,9 +35,6 @@ The kernel spawns the following system threads:
     and never terminates.
 
     The idle thread always uses the lowest configured thread priority.
-    If this makes it a cooperative thread, the idle thread repeatedly
-    yields the CPU to allow the application's other threads to run when
-    they need to.
 
     The idle thread is an essential thread, which means a fatal system error
     is raised if the thread aborts.