Doc: Use \value command for documenting flags for QML properties

Replace the table structure with \value commands for documenting
acceptable values for Dialog::standardButtons and
MessageDialog::standardButtons properties, and remove the
unnecessary \enum definitions.

Task-number: QTBUG-35019
Change-Id: Ifcb9ae8b2fbe6859fae324d855a395e4d1d224c1
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>