Whenever libcurl saves cookies to a file, it saves them in the old “Netscape cookie format”.
Since ages ago libcurl has written a comment at the top of that cookie file (the file we refer to as the cookie jar in curl lingo) that explains that it is a cookie netscape cookie file format and then a URL pointing to the original Netscape document describing what cookies are and how they work. To be perfectly honest, we pointed to a local copy of that document since the original is since long removed and gone and now only available through archive.org.
The web page pointed to were never a perfect match since the page never explained the file format or its use, only the cookie protocol – as it was originally described to work and not even like cookies works today.
Starting with curl 7.27.0, it will write the header to point to another URL: http://curl.haxx.se/docs/http-cookies.html