I was thinking more of an enterprise situation where multiple systems might be working together as part of a process. The code for each system might be in different repos in which case the documentation would also be broken. In such a context having the unit level documentation in the repo makes sense, but the overarching process documentation that is constantly evolving and referenced by multiple teams cannot be part of this repo.
The higher level strategic stuff can absolutely live elsewhere - in my experience Google Docs or some kind of company-wide wiki often come into play here.