Document use of colors with values greater than 1.0

## How can Bevy's documentation be improved?

> Values > 1.0 are allowed for linear colours and there is tonemapping code in the shaders that handles mapping HDR values where the colour value is > 1.0 to within 0.0 to 1.0. The bloom PR also specifically benefits from values > 1.0 and it will be common to set emissive values > 1.0 I think, in order for the light to bloom to the surroundings

From #3925.