Prior to this change any-domain cookies (cookies without a domain that
are sent to any domain) were exported with domain name "unknown".
Bug: https://github.com/bagder/curl/issues/292
Document that if Set-Cookie is used without a domain then the cookie is
sent for any domain and will not be modified.
Bug: http://curl.haxx.se/mail/lib-2015-05/0137.html
Reported-by: Alexander Dyagilev