Document point.show supporting callbacks

[PhrozenByte]

c3's point.show option supports callbacks, asking for every single point whether a point should be drawn or not. This feature isn't document yet.

https://github.com/c3js/c3/blob/1d496d312b6d6d3cdc43c0af048e8988adb84721/docs/js/c3.js#L1958