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.