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/Models/Response/PullRequestMerge.cs

@@ -4,9 +4,19 @@ using System.Globalization;
 
 namespace Octokit
 {
+    /// <summary>
+    /// Represents the response from an attempt to merge a pull request.
+    /// </summary>
+    /// <remarks>
+    /// Note the request to merge is represented by <see cref="MergePullRequest"/>
+    /// API: https://developer.github.com/v3/pulls/#merge-a-pull-request-merge-button
+    /// </remarks>
     [DebuggerDisplay("{DebuggerDisplay,nq}")]
     public class PullRequestMerge
     {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="PullRequestMerge"/> class.
+        /// </summary>
         public PullRequestMerge() { }
 
         public PullRequestMerge(string sha, bool merged, string message)