Tweaks to Core Mail inline docs

Will need an issue summary update to show how this is a problem.

Looking at the code "Unhandled \Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException " seems correct why use the other exception?

This came up again in BSI. I think we should rescope this issue to just fixing the @throws tag and split the $allowed_tags change into a Task as there is no bug there at the moment.

MailManager::getInstance should throw PluginException based on FactoryInterface::createInstance.