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/RepositoryIssueRequest.cs

@@ -2,6 +2,9 @@
 
 namespace Octokit
 {
+    /// <summary>
+    /// Used to requent and filter a list of repository issues.
+    /// </summary>
     [DebuggerDisplay("{DebuggerDisplay,nq}")]
     public class RepositoryIssueRequest : IssueRequest
     {