Mention the comment macro

[mishadoff]

Sometimes it useful to comment out multiple lines. That can be done with the comment macro:

(comment
  (some-code-1)
  (some-code-2)
  (some-code-3))

Maybe it's worth to mention that in style guide.

[bbatsov]

I think it's more important to mention the difference between #_ and comment and the fact that comment will actually return nil, which might break some code.