It's not documentation, but I've been using http://www.jetbrains.com/decompiler/ this to look at UnityEditor.dll and see how they do some of the custom editors themselves and have found it fairly educational.
You've got your pick of decompilers. Actually seeing the code isn't too tricky. The problem is the resulting decompiled code isn't compilable. This is intentional with at least one decompiler, but I've tried it with a paid solution and still haven't gotten immediately usable results.
You might have better luck than I did, but I couldn't find a method that wouldn't require several hours/days/weeks worth of manually recoding the bits the decompiler missed (and having to do so for every new release.)
Not sure if JustDecompile does that now but if you take JetBrains dotPeek, it will just fetch .NET Framework symbol information from MS symbol server and deconstruct the sources and let you browse them. No need to decompile when you have the source code, right?
Well, you can't really look up the actual source code, but since Terraria was written in .NET, you can get a pretty good approximation of what it probably looks like via decompiling/reflection.
I used JetBrains dotPeek; it's somewhat buggy and resource intensive, but it works and it's free.
I have used JetBrains dotPeek 1.1 and poked around a bit, and I came to the conclusion that the engine isn't where the major secrets are. But I may be wrong.
If you're still interested in delving I recommend using FEZ Parse, unless you already have it.
Regarding the public override void Update(GameTime gameTime){} method it's probably calling a platform-dependent code block, which is why System.Runtime.CompilerServices is imported in this particular class.
Out of curiosity, does using devexpress make your entire application closed source? You said it was just for the UI correct? Does that mean you could publish the DLLs out of curiosity?
Application looks very well done. I'm excited to use it.
EDIT: Also, for those interested, the famous ".NET Reflector" program (not reflection in VS) is no longer freeware. I hear DotPeek is pretty good though.