Update-rcconf-guide searches the package names corresponding to each service file in /etc/init.d directory from dpkg info files(/var/lib/dpkg/info/*.list) and get the description of these packages using apt-cache. Update-rcconf-guide uses Short-Description in priority to the description if service files has Short-Description field. Update-rcconf-guide writes those results to /var/lib/rcconf/guide.default file.
You can write your own guide in user guide file(/var/lib/rcconf/guide) by hand. Rcconf refers Guides in /var/lib/rcconf/guide before those in /var/lib/rcconf/guide.default.
If you install some packages after executed update-rcconf-guide, you need to re-create this file using update-rcconf-guide so as to refresh guide.default that includes new guides for installed new services.
Notice: update-rcconf-guide was not executed when you installed rcconf