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 iOS 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 Touch 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, you can obtain the updated version.
When you first install Xcode, the bulk of the documentation is not installed on your machine; viewing the documentation in the documentation window (discussed in the next section) may require an Internet connection, so that you can see the online docs at Apple’s site. This situation is untenable; you’re going to want a copy of the documentation locally, on your own machine.
Therefore, you should start up Xcode immediately after installation to let it download and install ...