`IrradianceVolume` has undocumented dependency on `LightProbe` to be functional

[kpreid]

https://docs.rs/bevy/0.16.1/bevy/pbr/irradiance_volume/index.html
https://docs.rs/bevy/0.16.1/bevy/pbr/irradiance_volume/struct.IrradianceVolume.html

Neither of these pages mentions that the entity having an IrradianceVolume component must also have a LightProbe component; I had to find that out by reading example code.

bevy/examples/3d/irradiance_volumes.rs

Lines 247 to 255 in ef71186

	commands.spawn((
	Transform::from_matrix(VOXEL_FROM_WORLD),
	IrradianceVolume {
	voxels: assets.irradiance_volume.clone(),
	intensity: IRRADIANCE_VOLUME_INTENSITY,
	..default()
	},
	LightProbe,
	));

This should be either documented, a required component, or both; I don't know which would be best.

[alice-i-cecile]

I think this should just be a required component: we're spawning this in with default parameters.