Hire a technical writer, proficient in the necessary domains, but without a stake in the design itself
If developers do their own documentation, it is a distraction from their "real" work. Hire a technical writer, proficient in the necessary domains, but without a stake in the design itself, to do the documentation.
The MercenaryAnalyst pattern is one that enables the practices of AgileModeling where documentation is necessary. A developer, according to the tenets of this Agile discipline, should model for a purpose, typically to understand the problem or to communicate the ideas to another developer. The model needs to be no more than adequate for the immediate purpose, and as a result the amount and rigour of models produced are considerably reduced. This brings considerable savings in effort. It may be the case that documentation is necessary, say for conveying the design of the system to future maintainers. More likely, the documentation is not really necessary but is mandated by the process or contract.