title: Which thread to use for performAsynchronousFileAccessUsingBlock when reading UIDocument incrementally
body: <p>I read a <code>UIDocument</code> incrementally and use its <code>performAsynchronousFileAccessUsingBlock</code> to facilitate this. Since I open the document on the main thread, I take care to also call this method on the main thread, but is that even necessary.</p>
  <p>So my question is this - does it matter on which thread I call <code>performAsynchronousFileAccessUsingBlock</code>?</p>