Document hcl functions

[droundy]

It would be lovely to have documentation for the argument ranges of the hcl color space, as there is for the other color spaces.

[mbostock]

Did you see the linked HCL Color Picker?

Hue is a degree, from 0° to 360°. Chroma is a percentage, from 0 to 100. I don’t know if the Luminance channel has a strict definition, since it depends on the reference illuminant (D65), other than 0 being black and higher values (up to 100-ish?) being brighter. But d3.hcl doesn’t protect you from creating colors that can’t be represented in RGB on standard hardware; see this post.