wordpress media library description column

Use the nl2br() function (tested):

function column_id_row( $columnName, $media_item ) {
    if ( 'description' !== $columnName ) {
        return;
    }

    $attachment_meta = get_post( $media_item );
    echo nl2br( $attachment_meta->post_content );
}
add_filter( 'manage_media_custom_column', 'column_id_row', 10, 2 );