Add some descriptions to new classes

Source/Core/VideoCommon/Present.h

@@ -23,6 +23,8 @@ namespace VideoCommon
 class OnScreenUI;
 class PostProcessing;
 
+// Presenter is a class that deals with putting the final XFB on the screen.
+// It also handles the ImGui UI and post-processing.
 class Presenter
 {
 public: