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/ReleaseUpdate.cs

@@ -4,14 +4,64 @@ using System.Globalization;
 
 namespace Octokit
 {
+    /// <summary>
+    /// Used to update a release.
+    /// </summary>
+    /// <remarks>
+    /// API: https://developer.github.com/v3/repos/releases/#create-a-release
+    /// </remarks>
     [DebuggerDisplay("{DebuggerDisplay,nq}")]
     public class ReleaseUpdate
     {
+        /// <summary>
+        /// Gets the name of the tag.
+        /// </summary>
+        /// <value>
+        /// The name of the tag.
+        /// </value>
         public string TagName { get; set; }
+
+        /// <summary>
+        /// Specifies the commitish value that determines where the Git tag is created from. Can be any branch or
+        /// commit SHA. Unused if the Git tag already exists. Default: the repository’s default branch
+        /// (usually master).
+        /// </summary>
+        /// <value>
+        /// The target commitish.
+        /// </value>
         public string TargetCommitish { get; set; }
+
+        /// <summary>
+        /// Gets or sets the name of the release.
+        /// </summary>
+        /// <value>
+        /// The name.
+        /// </value>
         public string Name { get; set; }
+
+        /// <summary>
+        /// Gets or sets the text describing the contents of the tag.
+        /// </summary>
+        /// <value>
+        /// The body.
+        /// </value>
         public string Body { get; set; }
+
+        /// <summary>
+        /// Gets or sets a value indicating whether this <see cref="NewRelease"/> is a draft (unpublished).
+        /// Default: false
+        /// </summary>
+        /// <value>
+        ///   <c>true</c> if draft; otherwise, <c>false</c>.
+        /// </value>
         public bool? Draft { get; set; }
+
+        /// <summary>
+        /// Gets or sets a value indicating whether this <see cref="NewRelease"/> is prerelease.
+        /// </summary>
+        /// <value>
+        ///   <c>true</c> if prerelease; otherwise, <c>false</c>.
+        /// </value>
         public bool? Prerelease { get; set; }
 
         internal string DebuggerDisplay