## Objective
Make it easier to use `IrradianceVolume` with fewer ways to silently
fail. Fix#19614.
## Solution
* Add `#[require(LightProbe)]` to `struct IrradianceVolume`.
* Document this fact.
* Also document the volume being centered on the origin by default (this
was the other thing that was unclear when getting started).
I also looked at the other implementor of `LightProbeComponent`,
`EnvironmentMapLight`, but it has a use which is *not* as a light probe,
so it should not require `LightProbe`.
## Testing
* Confirmed that `examples/3d/irradiance_volumes.rs` still works after
removing `LightProbe`.
* Reviewed generated documentation.
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