I found a nice PDF (100p) 2004-10-29 describing the Apache HTTP Server with many schema.
This web site offers results of the Apache Modeling Project. In this Project, research assistants and students of the Hasso-Plattner-Institute (HPI) have examined, modeled and documented the Apache HTTP Server.
I am still not convinced that FMC is more usefull than UML to describe high level concepts. But anyway, it is a great document.