sched: Clarify wake_up_q()'s write to task->wake_q.next

Clarify that wake_up_q() does an atomic write to task->wake_q.next, after
which a concurrent __wake_q_add() can immediately overwrite
task->wake_q.next again.

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250129-sched-wakeup-prettier-v1-1-2f51f5f663fa@google.com