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/MarkAsReadRequest.cs

@@ -4,17 +4,30 @@ using System.Globalization;
 
 namespace Octokit
 {
+    /// <summary>
+    /// Used to mark a notification as "read" which removes it from the default view on GitHub.com.
+    /// </summary>
+    /// <remarks>
+    /// https://developer.github.com/v3/activity/notifications/#mark-as-read
+    /// </remarks>
     [DebuggerDisplay("{DebuggerDisplay,nq}")]
     public class MarkAsReadRequest : RequestParameters
     {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="MarkAsReadRequest"/> class.
+        /// </summary>
         public MarkAsReadRequest()
         {
             LastReadAt = null;
         }
 
         /// <summary>
-        /// Describes the last point that notifications were checked. Anything updated since this time will not be updated.
+        /// Describes the last point that notifications were checked. Anything updated since this time will not be
+        /// updated.
         /// </summary>
+        /// <remarks>
+        /// This is sent as a timestamp in ISO 8601 format: YYYY-MM-DDTHH:MM:SSZ. Default: the current time.
+        /// </remarks>
         public DateTimeOffset? LastReadAt { get; set; }
 
         internal string DebuggerDisplay