Write documentation about d.o's project* configuration

Since project* has been designed with use on d.o in mind, there are sometimes features that don't make a lot of sense when used on smaller, less complicated sites. Since everyone has access to d.o, it would be good to give a detailed explanation of how project* on d.o is configured to work as it does. In this description it might be helpful to point out why certain choices of configuration are right for d.o but may be wrong for a smaller site.

I'm splitting this off from the original issue at http://drupal.org/node/193506