[ffigen] Dart API: Document the Dart API #2590

We've got quite a bit of documentation on https://github.com/dart-lang/native/tree/main/pkgs/ffigen#configurations, it would be good if the Dart API got the same amount of documentation.

What would be even better, is if all the options in the Dart API had snippets with the various options and how they impact the code gen. And we could ensure the snippets in the doc comments are exercised on the CI and we have a script that checks that the snippets in the doc-comments are identical to the ones tested.