doc: DistanceCIE76 underdocumented

[dolmen]

The documentation for DistanceCIE76 says:

That's actually the same, but I don't want to break code.

Same as what?

[ErnestoBorio]

As you can see in the code, it's the same that the function immediately before: (DistanceLab)
https://github.com/lucasb-eyer/go-colorful/blob/v1.0.3/colors.go#L594

[dolmen]

@ErnestoBorio My point is that should be written in the doc itself.

[KelSolaar]

Looking at the doc:

// DistanceLab is a good measure of visual similarity between two colors!

This is not quite true, simple Euclidean distance in CIE Lab space IS NOT a good measure because perceptual uniformity of CIE Lab is not great. It the main reason why the CIE introduced the CIE 1994 and CIE 2000 quasi-metrics.

[makew0rld]

@dolmen This is properly documented now.

@KelSolaar What would you suggest be written for DistanceLab instead?