dokuwiki-20130510a doesn't seem to install anything to /var/www/localhost/htdocs/.
Seems to use MY_HTDOCSDIR (which the eclass does as well), perhaps this is something you need to set in make.conf or package.env? Since I don't use web applications, assigned to the maintainers to help you further. Possibly something should output this directory during build or warn if it is not set or unsanely set?
The latest version seems to install just fine.