Refa: refine the context window size warning. (#5993)

### What problem does this PR solve?


### Type of change
- [x] Refactoring

rag/llm/chat_model.py

 import requests
 import asyncio
 
-LENGTH_NOTIFICATION_CN = "······\n由于长度的原因，回答被截断了，要继续吗？"
-LENGTH_NOTIFICATION_EN = "...\nFor the content length reason, it stopped, continue?"
+LENGTH_NOTIFICATION_CN = "······\n由于大模型的上下文窗口大小限制，回答已经被大模型截断。"
+LENGTH_NOTIFICATION_EN = "...\nThe answer is truncated by your chosen LLM due to its context length limitation."
 
 
 class Base(ABC):