document the custom render option

[Fil]

If we do an “Extending Plot” page, maybe it should cover more than the render function?

For #1711

[mbostock]

Should we also document the other public Mark methods, and add them to the Mark class TypeScript declaration? Specifically:

• the Mark constructor
• mark.initialize
• mark.filter
• mark.project
• mark.scale
• mark.render

(I also thought about all the public fields on Mark and subclasses, such as mark.data, mark.sort, mark.initializer, etc. But I don’t really want those to be part of the public API. 🤔)