Highlight faulty functionality of deprecated QPrinter::PrinterResolution

The documentation for QPrinter::PrinterResolution value says that this value is deprecated and that it’s equivalent to HighResolution on Windows and Mac. This however is not true for PDF printing. There the resolution should be set to 1200dpi on Windows when HihgResolution is used. But as the PrinterResolution is not handled at all on the code defining the resolution for PDF printing, using PrinterResolution is not equivalent to HighResolution:

QPdfPrintEnginePrivate::QPdfPrintEnginePrivate(…)
: …
{
    resolution = 72;
    if (m == QPrinter::HighResolution)
        resolution = 1200;
    else if (m == QPrinter::ScreenResolution)
        resolution = qt_defaultDpi();
}

The documentation should highlight the fact that using the deprecated value may lead to errors.