Update QThread's documentation so it doen't explicitly recommend to subclass QThread.

In the wave of this blog post:
http://labs.qt.nokia.com/2010/06/17/youre-doing-it-wrong/

It would be nice if Brad's good words could be reflected in the documentation, since most users and customers need to be able to trust it, and not necessarily follow Qt Labs Blogs.

Places where subclassing QThread is explicitly recommended:

http://doc.qt.nokia.com/4.7-snapshot/threads-starting.html
http://doc.qt.nokia.com/4.7-snapshot/qthread.html#details