sys-apps/dbus-1.4.12 installs the same api html in both: /usr/share/doc/dbus/api/ and: /usr/share/doc/dbus-1.4.12/html/api/
It also installs empty /usr/share/doc/dbus with USE "-doc"
+*dbus-1.4.14 (05 Aug 2011) + + 05 Aug 2011; Samuli Suominen <ssuominen@gentoo.org> +dbus-1.4.14.ebuild: + Version bump wrt #377603 by Sebastian Pipping. Fix API documentation building + and installing wrt #372293 by Chris Mayo.