You're falling victim to a common fantasy: that maintenance programmers will read great works of documentation before they set about making changes. In 20+ years in the business, I have rarely seen this happen. Instead, the best you can hope for is the equivalent of a quick glance at a map before hopping into the car, with occassional checking of the map along the way.

So go graphical, and give people something they can assimilate quickly. I like UML sequence diagrams for giving me a quick understanding of flow, and some form of class diagram to give me some idea how things are constructed. An occassional State chart is also an effective way of communicating what your program is doing.

Martin Fowler's UML Distilled is a excellent (and short) introduction to UML.