Going off on a tangent, but:
> This disappoints the ego-seeking behavior of many engineers. Good engineers often want people to realize how clever they were.
This is probably partly true to a small extent, but for the most part I think it trivialises a larger problem.
I have had this exact problem as an academic publishing papers. My ability to write straightforward easy to follow manuscripts has backfired on me many times, whereby explaining the steps in a clear, intuitive order has led reviewers to state that the findings were non-novel since they were a clear intuitive series of logical steps. My (sad) experience has taught me that if you make the reader 'work' a tiny bit towards developing the intuition themselves, and appreciate how this specific sequence of steps wasn't necessarily trivially obvious before announcing it on paper, then they will appreciate the novelty and the thinking that went behind it a lot more.
Obviously I don't necessarily think the mass of obfuscated papers out there are academics intentionally obfuscating to prevent rejection and raise appreciation in their readers (it's more likely to be a lack of skill or interest in investing the effort to write more clearly). But (despite it's frequent abuse as a phrase) leaving _some_ things as "an exercise to the reader" turns out to not always be the 'lazy' or 'ignorant' thing to do.
Now, of course, design documents may not necessarily have the same goals or gatekeepers as academic papers do, but I think attributing leaving some steps up to the reader to be entirely about engineering ego-seeking behaviour is a bit misguided. If your goal is to make someone understand and appreciate something, then there is such a thing as "explaining it too much" (and thus robbing them of the experience of developing their own, and far more useful, insight).
And this is doubly true in teaching contexts (arguably not the usual purpose of a design document, but ...).