title: QSettings clear, what does it do?
body: <p>The doc said about the <code>QSettings::clear</code> function as:</p>
  
  <blockquote>
    <p>Removes all entries in the primary location associated to this
    QSettings object.</p>
    
    <p>Entries in fallback locations are not removed.</p>
  </blockquote>
  
  <p>But what does this mean? what's the primary location and fall back location???</p>