X-Debbugs-Cc: Stefan Monnier <monnier@iro.umontreal.ca>

[ CC to Stefan, who seems to be the original author of that cookie
  handling thingy ... apologies if that does not really concern you ]

Variable `url-cookie-save-interval' can be perfectly well configured to
nil, as one can see in function `url-cookie-setup-save-timer' (which
seems to be the only function referencing that variable):

  (defun url-cookie-setup-save-timer ()
    "Reset the cookie saver timer."
    (interactive)
    (ignore-errors (cancel-timer url-cookie-timer))
    (setq url-cookie-timer nil)
    (if url-cookie-save-interval
        (setq url-cookie-timer (run-at-time url-cookie-save-interval
                                            url-cookie-save-interval
                                            #'url-cookie-write-file))))

However, the docstring and the :type of the defcustom do not
document/allow for nil as a valid value.

The attached patch fixes that, please check.

Thanks!