[docs] Clarify multi-scheduler acknowledgment behavior

[louiswang524]

The previous comment suggested only 1 scheduler process existed, which was misleading. In reality, world_size scheduler processes are spawned (one per TP rank/GPU), but only the primary rank sends an acknowledgment to the parent process.

This commit updates the comment to explicitly mention:

• world_size schedulers are created
• Only the primary rank sends an ack
• Total expected acks = 1 + num_tokenizers + 1