The Configuration Guide appears to be missing an important step for apache2.

Perhaps this is just a difference between using httpd and apache2 (and so the guide may well be OK), but for the record:

You need to create a phabricator.conf entry under /etc/apache2/sites-available/ that contains the <VirtualHost/> blurb that is in the guide.

Then you need to do something along the following lines:

sudo a2dissite 000-default
sudo a2ensite phabricator
sudo service apache2 restart

Finally, as an side, the part about mod_php being required doesn't seem to be true, at least for apache2 on Ubuntu.