Document INotification service and dispatcher

Author: shana

src/GitHub.Exports/Services/INotificationDispatcher.cs

@@ -24,6 +24,10 @@ public Notification(string message, NotificationType type, ICommand command = nu
         }
     }
 
+    /// <summary>
+    /// Dispatches notifications sent to the <see cref="INotificationService"/>
+    /// to registered listeners.
+    /// </summary>
     public interface INotificationDispatcher : INotificationService
     {
         IObservable<Notification> Listen();

src/GitHub.Exports/Services/INotificationService.cs

@@ -2,6 +2,10 @@
 
 namespace GitHub.Services
 {
+    /// <summary>
+    /// Service to broadcast messages, warnings and errors. Listeners
+    /// can receive them by registering with <see cref="INotificationDispatcher"/>
+    /// </summary>
     public interface INotificationService
     {
         void ShowMessage(string message);