Doc: Bring QSizeF/QPointF::isNull() documentation up to date

Since Qt 5.0 (commit 09dd19df

) sign is ignored when testing
whether a QPointF or QSizeF is null.

This updates the documentation accordingly.

Change-Id: I3de1c748f3caa63b8bd8990006de5ba572eac83e
Task-number: QTBUG-45669
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>