curl.1: mention the deprecated --krb4 option

docs/curl.1

@@ -957,6 +957,8 @@ This option requires a library built with kerberos4 support. This is not
 very common. Use \fI-V, --version\fP to see if your curl supports it.
 
 If this option is used several times, the last one will be used.
+.IP "--krb4 <level>"
+(FTP) This is the former name for \fI--krb\fP. Do not use.
 .IP "-l, --list-only"
 (FTP)
 When listing an FTP directory, this switch forces a name-only view. This is