docs: clarify SystemFrame queue behavior

[markbackman]

Summary

Updates the docs to clarify current SystemFrame queue behavior.

SystemFrames no longer bypass queues. They use the high-priority input queue, remain ordered with other SystemFrames, and are not discarded by interruptions. DataFrames and ControlFrames continue to use the non-system process queue and are ordered with each other.

Also adds the missing broadcast_sibling_id property to the Frames overview.

Fixes pipecat-ai/pipecat#4445

Changes

• Corrects stale “processed immediately” / “bypass queues” language in pipeline docs
• Clarifies ordering guarantees by processing lane
• Updates termination docs for CancelFrame priority behavior
• Updates RTVI docs for RTVIServerMessageFrame propagation behavior
• Documents broadcast_sibling_id in frame properties

Testing

• Searched docs for stale phrases like SystemFrames bypass queues, bypassing queues, processed immediately, and propagates immediately
• No docs build was run locally because the docs repo does not expose an obvious root build script

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