Document drag-threshold behavior of Flickable

Task-number: QTBUG-31168
Change-Id: Ia9452ad9f548b5eb9684603b4ab8b10f35710eae
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Andrew den Exter <andrew.den.exter@qinetic.com.au>