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/Request/NewCommitComment.cs

@@ -10,6 +10,10 @@ namespace Octokit
     [DebuggerDisplay("{DebuggerDisplay,nq}")]
     public class NewCommitComment
     {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="NewCommitComment"/> class.
+        /// </summary>
+        /// <param name="body">The body of the comment.</param>
         public NewCommitComment(string body)
         {
             Ensure.ArgumentNotNull(body, "body");