Add xmldoc to some high-level classes to explain their separation

osu.Game/OsuGame.cs

@@ -36,6 +36,10 @@ using osu.Game.Overlays.Volume;
 
 namespace osu.Game
 {
+    /// <summary>
+    /// The full osu! experience. Builds on top of <see cref="OsuGameBase"/> to add menus and binding logic
+    /// for initial components that are generally retrieved via DI.
+    /// </summary>
     public class OsuGame : OsuGameBase, IKeyBindingHandler<GlobalAction>
     {
         public Toolbar Toolbar;

osu.Game/OsuGameBase.cs

@@ -34,6 +34,11 @@ using osu.Game.Skinning;
 
 namespace osu.Game
 {
+    /// <summary>
+    /// The most basic <see cref="Game"/> that can be used to host osu! components and systems.
+    /// Unlike <see cref="OsuGame"/>, this class will not load any kind of UI, allowing it to be used
+    /// for provide dependencies to test cases without interfering with them.
+    /// </summary>
     public class OsuGameBase : Framework.Game, ICanAcceptFiles
     {
         protected OsuConfigManager LocalConfig;