Apparently, all session files are now *.desktop files stored inside /usr/share/xsessions. It doesn't seem like /etc/X11/Sessions is used anymore. This needs to be verified and, if it is correct, changed throughout all the docs. faq.xml gnome-config.xml kde-config.xml
Ideally, the policy that should/will be implemented is as follows: Each window manager will put a file in /etc/X11/Session _and_ a file in /usr/share/xsessions. Running 'startx' will still read the value of XSESSION form rc.conf, and run the correspondent file from /etc/X11/Session. kdm/gdm instead will provide a selection based on /usr/share/xsessions. But this is just an idea, and currently few WMs create the .desktop file in /usr/share/xsessions. And kdm will use /usr/share/xsessions only starting from kde-3.4.
I've updated gnome-config.xml to reflect this, and added a note about this in the kde-config guide.