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/ReferenceUpdate.cs

@@ -4,13 +4,28 @@ using System.Globalization;
 
 namespace Octokit
 {
+    /// <summary>
+    /// Upsed to update a Git reference.
+    /// </summary>
     [DebuggerDisplay("{DebuggerDisplay,nq}")]
     public class ReferenceUpdate
     {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ReferenceUpdate"/> class.
+        /// </summary>
+        /// <param name="sha">The sha.</param>
         public ReferenceUpdate(string sha) : this(sha, false)
         {
         }
 
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ReferenceUpdate"/> class.
+        /// </summary>
+        /// <param name="sha">The SHA1 value to set this reference to.</param>
+        /// <param name="force">
+        /// Indicates whether to force the update or to make sure the update is a fast-forward update. Leaving this
+        /// out or setting it to false will make sure you’re not overwriting work.
+        /// </param>
         public ReferenceUpdate(string sha, bool force)
         {
             Ensure.ArgumentNotNullOrEmptyString(sha, "sha");
@@ -19,7 +34,21 @@ namespace Octokit
             Force = force;
         }
 
+        /// <summary>
+        /// The SHA1 value to set this reference to.
+        /// </summary>
+        /// <value>
+        /// The sha.
+        /// </value>
         public string Sha { get; private set; }
+
+        /// <summary>
+        /// Indicates whether to force the update or to make sure the update is a fast-forward update. Leaving this
+        /// out or setting it to false will make sure you’re not overwriting work.
+        /// </summary>
+        /// <value>
+        ///   <c>true</c> if force; otherwise, <c>false</c>.
+        /// </value>
         public bool Force { get; private set; }
 
         internal string DebuggerDisplay