Add a bunch of doc comments

We get a lot of build output because of missing XML comments that we
ignore. I'd like to stop ignoring them. To do that, we need to doc the
💩 out of everything.

Octokit/Clients/MergingClient.cs

@@ -10,6 +10,10 @@ namespace Octokit
     /// </remarks>
     public class MergingClient : ApiClient, IMergingClient
     {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="MergingClient"/> class.
+        /// </summary>
+        /// <param name="apiConnection">The client's connection</param>
         public MergingClient(IApiConnection apiConnection) : base(apiConnection)
         {
         }