I often come across tips/pointers/exhortations about how to write good design documents. I generally agree that it's an important step: not just for clarifying your own thinking, but also for communicating effectively with others.

However, these types of posts often lack are concrete examples of what a good design document actually looks like. I understand that many of these documents are proprietary and intended for internal use. Still, are there any examples of well-written design documents available publicly that learners can study to get a clearer idea of what one should look like?