Knowledge is of two kinds. We know a subject ourselves, or we know where we can find information upon it.
You don't remember Cocoa; you look it up!
No aspect of Cocoa programming is more important than a fluid and nimble relationship with the documentation. There is a huge number of built-in classes, with many methods and properties and other details. Apple’s documentation, whatever its flaws, is the definitive official word on how you can expect Cocoa to behave and on the contractual rules incumbent upon you in working with this massive framework whose inner workings you cannot see directly.
The Xcode documentation installed on your machine comes in large chunks called documentation sets (or doc sets, also called libraries). You do not merely install a documentation set; you subscribe to it, so that when Apple releases a documentation update (because a new version of iOS has been released, or because there has been an incremental revision of the documentation), you can obtain the updated version.
When you first install the Xcode tools, assuming that you checked Documentation in the installer, you should start up Xcode to let it download and install your initial documentation sets. The process can be monitored, to some extent, in the Documentation pane of the Preferences window; you can also specify here whether you want updates installed automatically ...