Link between "QML Basic Type: date" and QML Date object documentation.

The basic date type seems useless if you don't know it can be used as
as JavaScript date (with QML extensions).

Task-number: QTBUG-32492

Change-Id: I14962f4e57522f5515f0e78d484aa59fcd9dff3c
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>