Improve on Mesh3d colorscale documentation

[VeraZab]

This should not be an attribute of mesh3d: https://plot.ly/javascript/reference/#mesh3d-colorscale

[mdtusz]

As discussed earlier, I'm not sure that it shouldn't be a part of it - it just needs better documentation, and very possibly, an implementation.

Talking briefly with @monfera earlier, it may be possible for us to modify the frag shader (or a wrapper around facecolor) to work using the z-position for coloring, but this will require someone a bit more knowledgable on our webgl front.

[mdtusz]

#643 has added a mock that uses mesh3d colorscale, but we could still improve upon the documentation so I will leave this open for the time being.

[etpinard]

Probably good enough after #1719