If you like parentheses, Scribble[1], a tool by the racket guys, does what you're asking for. I haven't used it extensively but the few times I've tried it, it has worked well. Here's an example scribble file[2].

[1]: http://docs.racket-lang.org/scribble/index.html [2]: http://git.racket-lang.org/plt/blob/HEAD:/collects/scribblin...