Document what happens if a message is reused when used on a batch

[doup]

$msg = $this->get('illarra_email.renderer')->createMessage(/* ... */);

$msg->setTo(['asier@example.com' => 'Asier']);
// Add to batch

// Update same message
$msg->setTo(['eneko@example.com' => 'Eneko']);

// Add to batch and send all emails

Document the result if something strange is found