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/GistFileUpdate.cs

@@ -4,11 +4,29 @@ using System.Globalization;
 
 namespace Octokit
 {
+    /// <summary>
+    /// Used as part of a <see cref="GistUpdate" /> to update the name or contents of an existing gist file
+    /// </summary>
+    /// <remarks>
+    /// API docs: https://developer.github.com/v3/gists/
+    /// </remarks>
     [DebuggerDisplay("{DebuggerDisplay,nq}")]
     public class GistFileUpdate
     {
+        /// <summary>
+        /// Gets or sets the new name of the file.
+        /// </summary>
+        /// <value>
+        /// The new name of the file.
+        /// </value>
         public string NewFileName { get; set; }
 
+        /// <summary>
+        /// Gets or sets the content.
+        /// </summary>
+        /// <value>
+        /// The content.
+        /// </value>
         public string Content { get; set; }
 
         internal string DebuggerDisplay