Swift

There's quite a lot of background required to even begin to understand this feature completely. However, the documentation here is to-the-point and definitely useful. I like this quite a lot because it also shows how to use the migration feature, which is cool and pretty much essential if you want to adopt this in an existing project.

Could also be quite eye-opening if you have been using concurrency with the compiler feedback disabled.

(This whole per-diagnostic/feature documentation effort is just great too.)