add-to-invisibility-spec adds element unconditionally

[martin rudalics]

From the Elisp manual:

 -- Function: add-to-invisibility-spec element
     This function adds the element ELEMENT to
     `buffer-invisibility-spec' (if it is not already present in that
     list). ...

But the element is added unconditionally:

(defun add-to-invisibility-spec (arg)
  ...
  (setq buffer-invisibility-spec
        (cons arg buffer-invisibility-spec)))

Also, the doc-strings of `add-to-invisibility-spec' and
`remove-from-invisibility-spec' should reference ARG.