Doc: Updated the C++ API changes list with QMotifStyle information

Task-number: QTBUG-31997

Change-Id: Iabe9f8f2d84a619031ff6bb3a846808419b36473
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>