ipvs: Document sysctl pmtu_disc

Document sysctl pmtu_disc based on commit 3654e611 ("ipvs: add
pmtu_disc option to disable IP DF for TUN packets").
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>

Documentation/networking/ipvs-sysctl.txt

@@ -175,6 +175,14 @@ nat_icmp_send - BOOLEAN
         for VS/NAT when the load balancer receives packets from real
         servers but the connection entries don't exist.
 
+pmtu_disc - BOOLEAN
+	0 - disabled
+	not 0 - enabled (default)
+
+	By default, reject with FRAG_NEEDED all DF packets that exceed
+	the PMTU, irrespective of the forwarding method. For TUN method
+	the flag can be disabled to fragment such packets.
+
 secure_tcp - INTEGER
         0  - disabled (default)