Blender Git Loki

Docs: Add more comments to geometry set header

This adds a bit more information to `GeometrySet` and each of the
geometry components. There is probably still more that can be written,
but this includes the most important information that I could think of.

I'd like to include some more general information about the
attribute API in a separate patch.

Differential Revision: https://developer.blender.org/D13501