I’m making a browser-based mmo. I’m hiring a developer, but I want to write up good specs for them. What do I speak about in the doc to give him all the details he needs to come up with a robust architecture?

The only thing I'm familiar with is the API concept. How do I delve into this further in the specs document? What other "components" of architecture should I list and delve into in the document?

Also, would anyone be open to sharing/posting a really good example of a software specs document? (For a product that is being built from scratch rather than a feature being added to an existing product.) I'd like to know what is articulated when the concept is in the idea stage and needs to take its first form in an mvp.