Include doc comments above `expect`s, not regular comments

[rtfeldman]

Today, if I write this:

# x should be positive
expect > 0

...that comment will appear in the failed expectation.

We talked about it on Zulip and concluded that comments shouldn't be this semantically meaningful. Instead, we should require using doc comments for this, like so:

## x should be positive
expect > 0

Now the doc comment is "documenting" the expect, and comments are back to being non-semantic.