Document piecewise/polylinear scales

[mbostock]

The documentation for continuous scales only covers two-element domains and ranges, but we support an arbitrary number of elements. This is most useful for color scales. We should document how this works.

Ref. #1633 (comment)

[Fil]

Revise this in light of #1715

[Fil]

duplicate of #2107 (or, the other way around), closed by #2112