I will start with a basic markdown outline and then use a prompt describing more of the system in just flowing (yet coherent) thought and, crucially, I'll ask the model to "Organize the spec in such a way that an LLM can best understand it and make use of it." The result is a much more succinct document with all the important pieces.

(or -- you can write a spec that is still more fleshed out for humans, if you need to present this to managers. Then ask the LLM to write a separate spec document that is tailored for LLMs)