This folder is called "web" in ispconfig and we dont plan to chnage it or make it configurable in the nextreleases. Just add a symlink "htdocs" in the web root that points to the "web" folder and your sites should work.
> Falko: can I proceed with a patch? Shall I create a new placeholder defaulting to "web"?
I'am not Falko, but I'am the maintainer of ispconfig.