bug#6151: mystery arguments of define-minor-mode + describe-function

[jidanni]

Some describe-function results add a mystery argument ARG, not visible
in the corresponding lisp code.

Or maybe it is visible, but not to your average reader.

E.g., here's what describe-function says

    longlines-mode is an interactive compiled Lisp function in
    `longlines.el'.

    (longlines-mode &optional ARG)

    Toggle Long Lines mode.
    In Long Lines mode, long lines are wrapped if they extend beyond

And here's where we get placed when clicking on longlines.el above

    ;;;###autoload
    (define-minor-mode longlines-mode
      "Toggle Long Lines mode.
    In Long Lines mode, long lines are wrapped if they extend beyond