Update outdated QThread docs to reflect current best practices

The docs suggest subclassing QThread. That used to be the only way to use QThread, but QThread's implementation changed significantly in Qt 4.4. Now, there's another way to use QThreads (see doc note in http://qt-project.org/doc/qt-5.0/qthread.html), which should be highlighted.

The developer responsible for QThread's evolution was going to update the docs (http://labs.qt.nokia.com/2010/06/17/youre-doing-it-wrong/#comment-7688), but it hasn't happened yet so I'm formally opening a ticket.