doc: Outdated comment in QTimer docs

In the QTimer docs, it reads "This is the traditional way of implementing heavy work in GUI applications, but as multithreading is nowadays becoming available on more and more platforms, we expect that zero-millisecond QTimer objects will gradually be replaced by QThreads"

This seems outdated and it partially contradicts the comment in the docs for QTimer::start(): "Keeping the event loop busy with a zero-timer is bound to cause trouble and highly erratic behavior of the UI."

So I think it's worth fixing up the documentation to remove the outdated comments, remove the recommendation of using zero-timers from the main docs and rather describe the current best practices/different tools we have for doing work while an event loop is still running.