Apple puts a lot of time and effort into maintaining its documentation. This documentation can often be a very valuable tool to determine how you are expected to interact with their frameworks.
Xcode actually integrates with the documentation quite well. One of the main ways you can look at the documentation is within the Quick Help inspector. You can display it by navigating to View | Utilities | Show Quick Help Inspector from the main menu. This inspector shows you the documentation of whatever piece of code you currently have your cursor on. If that particular class, method, or function is a part of Apple's frameworks, you will get some quick help with regards to it, as shown in the following screenshot:
Here the cursor is on UICollectionView, so the Quick Help inspector gives us the high-level information about it.
You can also look at the documentation in its own window if you need more information or want to do more exploring. You can open up this window at any time...