This is one of the reasons I tend to prefer comments over external documentation whenever possible. No, there's no magic force keeping comments in sync with the code they document changes, but you at least have a fighting chance. Programmers have enough state to juggle in their head as it is without expecting them to even necessarily know what all the external documentation it is they should update. Heck, even with comments it can be bad enough knowing whether some change to this module necessitates changes to comments in other modules in the code that you may have just looked at.

It isn't always possible. Very high-level architecture may belong in a format where diagrams are more available, and the high-level design justification for the project itself doesn't have a great place in the code.