I would encourage people to have one ARCHITECTURE file per directory of source code files. Don't duplicate documentation in these files - if there is a well documented header file for some module, just link to that.

In a big project, when I'm hunting for code that does something and I have no familiarity of the codebase, I want to be able to follow a chain of ARCHITECTURE documents from the root of the project to the implementation of the feature I'm looking for.

If your documentation is poor enough that I resort to finding a string used in the feature and grepping the whole source tree for it, then your codebase will be tricky for someone new to get started on.