[IPVS]: Document the ports option to ip_vs_ftp in kernel-parameters.txt

I'm not sure if documenting this here is appropriate, but
if it is, here is some text to put there.
Signed-Off-By: NSimon Horman <horms@verge.net.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/kernel-parameters.txt

@@ -697,6 +697,12 @@ running once the system is up.
 	ips=		[HW,SCSI] Adaptec / IBM ServeRAID controller
 			See header of drivers/scsi/ips.c.
 
+	ports=		[IP_VS_FTP] IPVS ftp helper module
+			Default is 21.
+			Up to 8 (IP_VS_APP_MAX_PORTS) ports
+			may be specified.
+			Format: <port>,<port>....
+
 	irqfixup	[HW]
 			When an interrupt is not handled search all handlers
 			for it. Intended to get systems with badly broken