Document reflection limitations for references

## How can Bevy's documentation be improved?

Based on discussion in #21282, we might want to clarify in the docs that reflection does not have out-of-the-box support reflecting references yet beyond certain types like `&'static str` and `&'static Path`.

I think we could put this under the [`Limitations`](https://docs.rs/bevy/0.18.1/bevy/reflect/#limitations) section or perhaps even as part of its [`Non-Static Lifetimes`](https://docs.rs/bevy/0.18.1/bevy/reflect/#non-static-lifetimes) sub-section since it's somewhat related.