Although most of the good Android libraries are open source, occasionally you get stuck with a third party JAR or two that doesn't have source available, or good documentation, or both.
Fortunately, there is a fantastic IDEA plugin to solve this problem called JD-IntelliJ. Android Studio appears to be fully compatible with IDEA plugins and this one works very well. It's not faultless and a small number of methods are output as "described byte code" but this itself isn't too tricky to follow.
Here is a quick example taken from Deezer's library: