Vector docs

Documents relevant changes for the vectors refactor in https://github.com/Leaflet/Leaflet/issues/3098.

I'm making a conscious choice to not document `L.Renderer`. I doubt anyone will ever need to use it and very few people (probably no one) will extend it, so i dont think it needs to be documented, especially for a 1.0-beta1

I removed `L.MultiPolygon` and `L.MultiPolyline from the doc entirely in favor of more usage examples for `L.Polyline` and `L.Polygon` demonstrating the single and multi variants..

This should probably be reviewed by someone involved in the vector refactor, most likely @mourner @danzel or @jacobtoye.