Document starlark API better

[pcj]

The ability to implement plugins in starlark is likely the preferred way to implement a plugin since it works out of the box with proto_repository, but the API is poorly documented. I myself, who wrote it, have to review the code at a fairly low-level to remember how to implement a plugin each time I do it.

Come up with a way to annotate the go structs such that we can generate the API documentation programmatically.