[Doc] QStringConverter lacks ICU support information

From developer point of view who uses Qt6 its unclear about text en/decoding support in Qt.  You can't find proper information how to deal with codepages other than Unicode and Latin-1.

Old QTextCodec was moved Qt5Compat and there is no information about recently added ICU backend to QStringConverter.

https://doc.qt.io/qt-6/qstringconverter.html

And icu support hidden behind QStringConverter::Flag::UsesIcu flag? It works automatically or developer should set that flag explicitly? None such information available.

https://codereview.qt-project.org/c/qt/qtbase/+/417200/3/src/corelib/text/qstringconverter.cpp