Doc: Update details on VS Add-In support

Task-number: QTBUG-33360

Change-Id: I40d97a66d2047d1121b1a0fcf8ef8c19c9a7bba5
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>