Add gamut mapping details

[palemieux]

From https://github.com/w3ctag/design-reviews/issues/917#issuecomment-1964765613

While we appreciated the discussion on volume mapping, we still had questions about the specifics. We see several cases where gamut/volume/tone mapping come into play, and we’d love some more detail on how these are handled (actually, the same questions apply to wider/smaller gamuts too):

• HDR image painted on a non-HDR canvas
• HDR canvas displayed non-HDR screen
• Author painting an HDR CSS color on a non-HDR canvas

[palemieux]

Annex A already contains many details. I have asked for clarification on the comment at https://github.com/w3ctag/design-reviews/issues/917#issuecomment-2179420173