There is no way to add a product image description using the addImageToMediaGallery function. The following code does the job though:

foreach($product->getData('media_gallery') as $each){
    foreach($each as $image){
         $attributes = $product->getTypeInstance(true)->getSetAttributes($product);
         $attributes['media_gallery']->getBackend()->updateImage($product, $image['file'], array('label' => 'whatever'));
     }
}