Document (or make private) QPaintDevice::encodeMetricF(PaintDeviceMetric metric, double value)

int QPaintDevice::encodeMetricF(PaintDeviceMetric metric, double value) is new public API in Qt 6.8, introduced in https://codereview.qt-project.org/c/qt/qtbase/+/560069 and mentioned in that change log.

However, as pointed out in header review, it is not documented (not even as \internal).

Decide what to do with that API before 6.8 RC. Options could be:

• keep as public static function, but document
• document as \internal
• make it an internal qt_encodeMetrics() function
• make it a private function and add friends where needed