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/PullRequestRequest.cs

@@ -5,6 +5,9 @@ using Octokit.Internal;
 
 namespace Octokit
 {
+    /// <summary>
+    /// Used to filter requests for lists of pull requests.
+    /// </summary>
     [DebuggerDisplay("{DebuggerDisplay,nq}")]
     public class PullRequestRequest : RequestParameters
     {