[css-color-5] Add missing groups in <light-dark-image> syntax

[cdoublev]

Follow-up on 100146f.

https://un5n798jx6qx6j0rmf2verhh.julianrbryant.com/css-color-5/#typedef-light-dark-image

@yisibl, <image> | none must be grouped because <image> | none , <image> | none is equivalent to <image> | [ none , <image> ] | none.

Juxtaposition is stronger than the double ampersand, the double ampersand is stronger than the double bar, and the double bar is stronger than the bar.

https://un5n798jx6qx6j0rmf2verhh.julianrbryant.com/css-values-4/#component-combinators

[yisibl]

Thanks for spotting that! I saw the [] inside the commented-out <!-- <pre class='prod'> section and assumed it was already there.