Documentation of public methods on private structs is hidden

It seems like a lot of the API documentation is not visible on go.dev or godoc.org because of the fact that these methods belong to private structs used as mixins into public structs.

So for example the `*Application` returned by `kingpin.New` has a `Flag` method provided by its embedded `cmdMixin`, which in turn embeds a `*flagGroup`. But this method is entirely missing from the API docs because `flagGroup` is private.

I don't know that this is easily fixable in v2 but it'd be really nice to fix this in v3.