Clearer documentation about and/or support for multipart features

[JamesHovet]

Multi-part features, a fairly common occurrence in the GIS world, are not mentioned in the documentation at all. You can certainly draw them, but it is unclear how they behave in other situations.

Perhaps we should add more explicit support for operations with them, but at the very least we should make reference to them in the documentation so that people are not flying blind.