ipvs: Document sysctl sync_ports

Document sysctl sync_ports based on commit f73181c8 ("ipvs: add support
for sync threads").
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>

Documentation/networking/ipvs-sysctl.txt

@@ -231,6 +231,14 @@ sync_sock_size - INTEGER
 	Configuration of SNDBUF (master) or RCVBUF (slave) socket limit.
 	Default value is 0 (preserve system defaults).
 
+sync_ports - INTEGER
+	default 1
+
+	The number of threads that master and backup servers can use for
+	sync traffic. Every thread will use single UDP port, thread 0 will
+	use the default port 8848 while last thread will use port
+	8848+sync_ports-1.
+
 snat_reroute - BOOLEAN
 	0 - disabled
 	not 0 - enabled (default)