Add ability to force-show certain undocumented types even with `skip_undocumented`

[sindresorhus]

I want to use skip_undocumented: true to skip undocumented types. However, I have a Swift enum with a large amount of casees and they don't really need to be documented as their name makes it clear enough. It would be nice to be able to use some kind of code comment to indicate that I don't want to exclude those undocumented enum cases (without having to add it to each case).

[johnfairh]

Depending on scale you may find it easier to turn off skip_undocumented and use :nodoc: to explicitly opt items out of documentation.

[sindresorhus]

That's my current workaround, yes. But I now have a lot of those comments all over the place.