I did an "GENTOO_SRCDIR=/usr/gentoo-src emerge gentoo-web" to install a local copy of the gentoo web site, then an "emerge boa" to install boa-0.94.11-r2. I edited /etc/boa/boa.conf to have Port 8080 and DocumentRoot /www/virtual/www.gentoo.org/htdocs. Pointing galeon at http://localhost:8080 brings up my local gentoo page just fine. Clicking on "About Gentoo Linux" works fine, as does clicking on any of the "Recent Gentoo Linux news" items. In fact, "http://localhost:8080/news/" gives me a listing of the news entries. Clicking on any of the "User Doc" entries, though, gives a 404 Not Found. Entering "http://localhost:8080/doc/" reveals the problem, as what's listed is the contents of /usr/share/doc, which isn't at all what I'd had in mind (and also not at all below the DocumentRoot). Huh?!
There is an alias for /doc which points to /usr/share/doc Simply remove the alias. Note that this behavior is consistent with Aliases being devirtualized before consulting the file system, which is a design consideration, and the same consideration made by all other httpds I have consulted.