curl.1: --disable-{eprt,epsv} are ignored for IPv6 hosts

The behavior has been clarified in CURLOPT_FTP_USE_{EPRT,EPSV}.3 man
pages since curl-7_12_3~131.  This patch makes it clear in the curl.1
man page, too.

Bug: https://bugzilla.redhat.com/1305970