They already started doing it a few months ago with private annotations. This "nonnull"/"nullable" syntax is a result of their work on annotating Cocoa APIs.
They've been doing it for a while in the Swift overlay module. I'm really happy that this is officially added to obj-c because it provides very valuable API documentation, even if it won't have much practical effect on the code.
I assume that, now that this feature exists, Apple will begin annotating their Obj-C APIs.