Document colorspace specs

[flibitijibibo]

Probably makes sense to give more details here, that it's Windows scRGB, etc:

          The expected mapping is such that a color value of (1.0, 1.0, 1.0)
          corresponds to a luminance level of 80 nits on a standardized studio
          monitor.
          As the color value per channel goes beyond 1.0 and up to 125.0, the
          corresponding luminance levels also increase linearly to
          a maximum of 10000 nits.

Ref https://github.com/libsdl-org/SDL/pull/9312/files#r1711938621