Document how to use Locale type in QQmlExpression

See the workaround done here:

https://codereview.qt-project.org/c/qt/qtdeclarative/+/335964/4..5

It's not possible to evaluate the following QQmlExpression:

locale.formattedDataSize(1000000, 3, Locale.DataSizeIecFormat)

in this context:

https://code.qt.io/cgit/qt/qtdeclarative.git/tree/tests/auto/qml/qqmllocale/data/functions.qml?id=b3567c177f231f28e8bee32d54263b24148c722b

I've tried removing import version numbers and adding import QtQml, but neither helped.

Minimal example attached.