I'm not really a fan of this literate coffeescript. Most of the code speaks for itself, and the English explanation make the code much harder to follow. It's worse than "add one to i" because at least that doesn't take up multiple lines.
I agree that Underscore's functions are simple/short enough that the commentary doesn't add much. However, do you still think that literate-style documentation isn't helpful for more complicated modules like this one?
Those English explanations are just the comments from the original JavaScript version, put in their own column. If anything, this avoids breaking the flow of the code itself.
BTW, these beautiful documentation webpages are auto-generated with a tool, as pointed out in the underscore.js post: http://news.ycombinator.com/item?id=1767897
There are equivalents for several languages.