Some teams at Microsoft do have internal documentation. We decided about 10 years ago, before our product got away from us, to start documenting the architecture. This is something that our management completely supported (they may have even suggested it), and we added this effort to our schedule, not as a "slack time" thing. Turns out that forcing yourself to document the architecture also helps keep the architecture sane because it really hurts to document something ugly.

We also have a ton of specs, archived off over the last 15 years. Those can sometimes be quite useful.