HDR and sRGB Values in Bevy

While studying the Bevy engine and the [2D Bloom example](https://github.com/bevyengine/bevy/blob/latest/examples/2d/bloom_2d.rs), I noticed that the example compiles and works fine, but the documentation is unclear about handling large color values, especially with HDR.

A similar issue was raised over a year ago:

> I guess at this point I would be happy if there was a small mention in the documentation that large values are allowed with HDR, and `Mul` implementation for Srgb and LinearRgb to manipulate HDR brightness (palette has this too).
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> All color space conversions may not work very well with HDR but maybe it doesn't matter if they weren't even designed for that.

_Originally posted by @eero-lehtinen in [#12089](https://github.com/bevyengine/bevy/issues/12089#issuecomment-1962844451)_

I suggest updating the documentation to clarify how HDR works with large values.
#20281

Start with the 2D Bloom example at examples/2d/bloom_2d.rs and review the referenced discussion in issue #12089. Update the relevant documentation to explain how HDR permits large color values and clarify the expected handling of HDR brightness and color-space conversions.

Written by the indexing model from the issue text.