Document the fact that QPA is experimental in Qt 4.8

Task-number: QTBUG-26912

Change-Id: I8544a970f58faf9fd9811603d6640ad2b99ebe1d
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>