doc: update http.cookieFile with in-memory cookie processing
Documentation only mentions how to read cookies from the given file
and how to save them to the file using http.saveCookies.
But underlying libcURL allows the HTTP cookies used only in memory;
cookies from the server will be accepted and sent back in successive
requests within same connection, by using an empty string as the
filename. Document this.
Signed-off-by: Piotr Szlazak <piotr.szlazak@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>