I know a lot of managers feel that a system should have a certain amount of documentation supporting it, but in my experience often
documentation that is produced for this purpose never gets used after it is written. I would suspect some sort of high level design of the system which describes it's interfaces, major architectural components and design patterns is useful. Perhaps also a support/operations manual for those that need to support the system in production can be useful. However when it gets down to detailed design documents such as class diagrams and sequence diagrams I rarely find them useful. Usually they don't get maintained and they just don't help you understand the system that much.
Although I'm not totally convinced I'm starting to lean towards the idea that good software is self documenting and software quality is more important than documentation. Also having been involved in some agile projects lately, I'm starting to see how the automated tests for a system can be the best documentation of what the system does.