Add output to subsets docs examples

I find that the code examples in the docs are hard to follow. You can try the code in the REPL easily enough, but when I look at documentation, I want to know what the code does as I read it. That way, the code I explore in the REPL can be more focused, since I've identified something close to what I want by reading the documentation examples.

I'm happy to make this change to the other sections of the docs, but I wanted to check if this a good direction to go in. Is this formatting acceptable? Are there reasons not to include output for examples in the documentation?