[doc] RAGFlow's api key never expires (#1188)

### What problem does this PR solve?

_Briefly describe what this PR aims to solve. Include background context
that will help reviewers understand the purpose of the PR._

### Type of change

- [x] Documentation Update

docs/guides/llm_api_key_setup.md

 - [VolcEngine](https://www.volcengine.com/docs/82379).
 
 :::note
-If you find your online LLM is not on the list, don't feel disheartened. The list is expanding, and you can [file a feature request](https://github.com/infiniflow/ragflow/issues/new?assignees=&labels=feature+request&projects=&template=feature_request.yml&title=%5BFeature+Request%5D%3A+) with us! Alternatively, if you have customized models or have locally-deployed models, you can [bind them to RAGFlow using Ollama or Xinference](./deploy_local_llm.md).
+If you find your online LLM is not on the list, don't feel disheartened. The list is expanding, and you can [file a feature request](https://github.com/infiniflow/ragflow/issues/new?assignees=&labels=feature+request&projects=&template=feature_request.yml&title=%5BFeature+Request%5D%3A+) with us! Alternatively, if you have customized or locally-deployed models, you can [bind them to RAGFlow using Ollama or Xinference](./deploy_local_llm.md).
 :::
 
 ## Configure your API key

docs/references/api.md

 
 ## Authorization
 
-All of RAGFlow's RESTFul APIs use API key for authorization, so keep it safe and do not expose it to the front end. 
+All of RAGFlow's RESTful APIs use API key for authorization, so keep it safe and do not expose it to the front end. 
 Put your API key in the request header. 
 
 ```buildoutcfg
 Authorization: Bearer {API_KEY}
 ```
 
+:::note
+In the current design, the RESTful API key you get from RAGFlow does not expire.
+:::
+
 To get your API key:
 
 1. In RAGFlow, click **Chat** tab in the middle top of the page.