sched/core: Update kernel boot parameters for LAZY preempt.

Update the documentation for the `preempt=' parameter which now also
accepts `lazy'.

Fixes: 7c70cb94 ("sched: Add Lazy preemption model")
Reported-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Link: https://lore.kernel.org/r/20241122173557.MYOtT95Q@linutronix.de