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

docs/curl.1

@@ -430,6 +430,9 @@ the traditional PORT command.
 \fB--eprt\fP can be used to explicitly enable EPRT again and \fB--no-eprt\fP
 is an alias for \fB--disable-eprt\fP.
 
+If the server is an IPv6 host, this option will have no effect as EPRT is
+necessary then.
+
 Disabling EPRT only changes the active behavior. If you want to switch to
 passive mode you need to not use \fI-P, --ftp-port\fP or force it with
 \fI--ftp-pasv\fP.
@@ -441,6 +444,9 @@ but with this option, it will not try using EPSV.
 \fB--epsv\fP can be used to explicitly enable EPSV again and \fB--no-epsv\fP
 is an alias for \fB--disable-epsv\fP.
 
+If the server is an IPv6 host, this option will have no effect as EPSV is
+necessary then.
+
 Disabling EPSV only changes the passive behavior. If you want to switch to
 active mode you need to use \fI-P, --ftp-port\fP.
 .IP "--dns-interface <interface>"