kern/thread: add comment about balancer threads

Balancer threads use their own version of thread_sleep because they have to
keep the run queue locked after pulling threads, otherwise remote balancers
might pull threads from a run queue right before a scheduling decision is
made, possibly putting the run queue in a state where only expired threads
remain.

Although this might be relaxed in the future, this is how it's currently
done, and a clear note in the main balancer function definitely helps against
breaking that part.

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