Update dialog helper docs to add 2.3 features

[wouterj]

As of 2.3, the DialogHelper::select method accepts multiselect by using comma seperated values:

$dialog = $app->getHelperSet()->get('dialog');
$colors = array('red', 'blue', 'yellow');

$selected = $dialog->select(
    $output,
    'Please select your favorite color (default to red)',
    $colors,
    0,
    false,
    'Value "%s" is invalid',
    true // multiselect support is disabled by default
);

$selectedColors = array_map(funtion($c) use ($colors) {
    return $colors[$c];
}, $selected)

$output->writeln('You have just selected: ' . implode(', ', $selectedColors));

Now, when a user inputs 1,2, the result will be "You have just selected: blue, yellow".

[wouterj]

related PR: symfony/symfony#7602