onlyOperationTypes documentation

[tpict]

Moving some client-side code over from Apollo tools, I was having issues with duplicated types for documents like

mutation DoSomething {
  doSomething {
    ok
  }
}

because GCG was creating a complete DoSomethingMutation type for the field in the schema, and another DoSomethingMutation type for this particular document.

The solution is to set onlyOperationTypes: true, so it doesn't emit the former. But I only found that out through dumb luck, because the documentation says:

This will cause the generator to emit types for operations only (basically only enums and scalars)

What does that mean? What do enums/scalars have to do with this setting?

I'd open a PR to put something more descriptive here, but want to make sure this setting does what I think it does.