As noted in Documentation error, there is an error in the mailer documentation. Although a fix was made in the repository and that report closed back in November, it seems the documentation on secure.phabricator.org has not yet been regenerated with the change.
secure.phabricator.org documentation should be regenerated and checked before closing reports about documentation errors.
Refer to secure.phabricator.org
View the documentation at https://secure.phabricator.com/book/phabricator/article/configuring_outbound_email/