bug#5565: dired-at-point docstring prefix argument

[jidanni]

In ffap.el we see:
    (defcustom dired-at-point-require-prefix nil
      "If set, reverses the prefix argument to `dired-at-point'.
    This is nil so neophytes notice ffap.  Experts may prefer to disable
    ffap most of the time."

Well, OK, but

    (defun dired-at-point (&optional filename)
      "Start Dired, defaulting to file at point.  See `ffap'."

then should really mention what its prefix argument does.