Add Docs for `retrieval_strategy` in the Query Agent's Search Mode

### Code of Conduct

- [x] I have read and agree to the Weaviate's [Contributor Guide](https://docs.weaviate.io/contributor-guide) and [Code of Conduct](https://weaviate.io/service/code-of-conduct)

### What part of document/web-page on weaviate.io is affected?

Query Agent

### Additional comments?

_No response_