Make the use case for configuring `TimeUpdateStrategy` and `Time<Real>` more clear

## How can Bevy's documentation be improved?

https://github.com/bevyengine/bevy/issues/15409#issuecomment-2377332228

The documentation should make it clear that this is to enable people to write test cases that mock wall time.

Would it be possible to create a warning when this is attempted outside of `#[cfg(test)]`?
