(defun foo ()
  "Doc string of foo.
If you have to quote a parenthesis at the bol, it will be trouble for
elisp-eldoc-documentation-function.
Write (like this) and you will find no problem.  But put
\(a quoted parenthesis) and you'll confuse elisp-eldoc-documentation-function."
  nil)

;; Also, in comments, look what happens when you put a " here.