title: What is the difference between NSURLRequest's mainDocumentURL and URL properties?
body: <p>I've built a web browser for iOS and I'm trying to pull the URL to display in an address bar.<br>
  I look at the <code>NSURLRequest</code> in the <code>webView:shouldStartLoadWithRequest:navigationType:</code> and the <code>webViewDidFinishLoad:</code> <code>UIWebView</code> delegate methods. </p>
  
  <p>Should I use the <code>URL</code> or the <code>mainDocumentURL</code> property of the <code>NSURLRequest</code> for that purpose, and what is the difference?</p>