Improve documentation on constructing the client

The documentation doesn't describe how the client is constructed using the database name, resource ARN and secret ARN. This seemed to have changed because [this](https://stackoverflow.com/questions/66145341/aurorards-serverless-with-data-api-library-in-java-does-not-work/70902387#70902387) example code doesn't work anymore because `RdsDataClient` no longer contains the described methods. I've searched a long time for examples but I'm unable to get the client to work.

首先检查仓库现有的文档和当前的 RdsDataClient API。记录如何使用数据库名称、资源 ARN 和 secret ARN 构造客户端，并包含一个反映当前可用方法的可运行示例。

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