Add comments to concrete types

We now have a build task that copies comments from interface to
implementation.

Octokit/Clients/GitDatabaseClient.cs

@@ -1,5 +1,11 @@
 namespace Octokit
 {
+    /// <summary>
+    /// A client for GitHub's Git Database API. Gives you access to read and write raw Git objects and to list and update your references.
+    /// </summary>
+    /// <remarks>
+    /// See the <a href="http://developer.github.com/v3/git/">Git API documentation</a> for more information.
+    /// </remarks>
     public class GitDatabaseClient : ApiClient, IGitDatabaseClient
     {
         /// <summary>