Improve documentation of custom QDebug stream operators

The original example was resetting the stream to space().
Since Qt 5.1 we have QDebugStateSaver to avoid this, so let's use it.

Change-Id: Ia840a9bad121c0f7bc6714742f58b52c59fb7c74
Reviewed-by: David Faure <david.faure@kdab.com>