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>

Author: pszlazak

Documentation/config/http.txt

@@ -82,12 +82,16 @@ http.cookieFile::
 	in the Git http session, if they match the server. The file format
 	of the file to read cookies from should be plain HTTP headers or
 	the Netscape/Mozilla cookie file format (see `curl(1)`).
+	Set it to an empty string, to accept only new cookies from
+	the server and send them back in successive requests within same
+	connection.
 	NOTE that the file specified with http.cookieFile is used only as
 	input unless http.saveCookies is set.
 
 http.saveCookies::
 	If set, store cookies received during requests to the file specified by
-	http.cookieFile. Has no effect if http.cookieFile is unset.
+	http.cookieFile. Has no effect if http.cookieFile is unset, or set to
+	an empty string.
 
 http.version::
 	Use the specified HTTP protocol version when communicating with a server.