is_final_response() is not correctly used and described.

**Describe the bug**

Below statement is not true when it's multi-agent setup. `is_final_response` only means it's the last message by the agent, it may be identical to the last message of the turn in a single-agent setup, but it's not true in most case for multi-agent setup.

```
# Key Concept: is_final_response() marks the concluding message for the turn.
if event.is_final_response():
```

Also `is_final_response` is a framework internal concept. We'd better not expose it in docs.