QAbstractFormBuilder::save(): Add a note explaining property issues

State that the resulting files are not intended to be used
without editing.

Change-Id: I5ae342907a9394427ef9d3471ff2670421da8cc0
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Kavindra Palaraja <kpalaraja@luxoft.com>