Docs: add a working example to Text::fontSizeMode docs

It's easy to miss the maximum bound specified by either
the font.pointSize or font.pixelSize properties.

Task-number: QTBUG-30005
Change-Id: If1dadebd6673f0e945a1ca95b64521f27d30f5a9
Reviewed-by: Alan Alpert <aalpert@blackberry.com>